[YSQL] Implement UNIQUE constraint

[mbautin]

E.g. needed by Django support -- a default new project schema requires it:

11:10 $ ./manage.py migrate
/home/mikhail/test/django/venv/lib/python3.6/site-packages/psycopg2/__init__.py:144: UserWarning: The psycopg2 wheel package will be renamed from release 2.8; in order to keep installing from binary please use "pip install psycopg2-binary" instead. For details see: <http://initd.org/psycopg/docs/install.html#binary-install-from-pypi>.
  """)
Operations to perform:
  Apply all migrations: admin, auth, contenttypes, sessions
Running migrations:
  Applying contenttypes.0001_initial...Traceback (most recent call last):
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
    return self.cursor.execute(sql, params)
psycopg2.NotSupportedError: UNIQUE constraint not supported yet
LINE 1: ...jango_content_type_app_label_model_76bd3d3b_uniq" UNIQUE ("a...
                                                             ^
HINT:  Please contact YugaByte for its release schedule.


The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "./manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/core/management/__init__.py", line 375, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/core/management/base.py", line 316, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/core/management/base.py", line 353, in execute
    output = self.handle(*args, **options)
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/core/management/base.py", line 83, in wrapped
    res = handle_func(*args, **kwargs)
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/core/management/commands/migrate.py", line 203, in handle
    fake_initial=fake_initial,
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/db/migrations/executor.py", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/db/migrations/executor.py", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/db/migrations/executor.py", line 244, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/db/migrations/migration.py", line 124, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/db/migrations/operations/models.py", line 514, in database_forwards
    getattr(new_model._meta, self.option_name, set()),
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 360, in alter_unique_together
    self.execute(self._create_unique_sql(model, columns))
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 133, in execute
    cursor.execute(sql, params)
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/db/backends/utils.py", line 100, in execute
    return super().execute(sql, params)
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/db/backends/utils.py", line 68, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/db/backends/utils.py", line 77, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
    return self.cursor.execute(sql, params)
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/db/utils.py", line 89, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/home/mikhail/test/django/venv/lib/python3.6/site-packages/django/db/backends/utils.py", line 85, in _execute
    return self.cursor.execute(sql, params)
django.db.utils.NotSupportedError: UNIQUE constraint not supported yet
LINE 1: ...jango_content_type_app_label_model_76bd3d3b_uniq" UNIQUE ("a...
                                                             ^
HINT:  Please contact YugaByte for its release schedule.

[frozenspider]

Implemented in e283667