feat: Upgrade SQLAlchemy v1.4 for native asyncio support

[achimnol]

Internal ticket: OP#1161

Major changes:

• Drop references to aiopg and psycopg2 and add asyncpg as our dependency
• Update imports and aliases for SAConnection, SAEngine (previously SAPool)
  • SQLAlchemy's engine includes the connection pool interface.
  • RowProxy is deprecated and replaced with sa.engine.row.Row
  • Row still supports __getitem__() interface but we need to use Row._mapping if we need other dict-like interfaces such as .get(key, default).
• async with dbpool.acquire() as conn: --> async with dbpool.connect() as conn:
• await result.{fetchall,scalar,first}() --> result.{fetchall,scalar,first}() because the result is already buffered by await conn.execute()
  • To asynchoronously stream the result from the server, use await conn.stream(query) instead of await conn.execute(query)
• Remove deprecated .as_scalar() and use .scalar_subquery() for scalar-valued subqueries
• Do NOT use the JIT compilation of high-cost queries in PostgreSQL v11+ to prevent excessive delay on connection setup due to asyncpg's type introspection queries (Slow introspection when using multiple custom types in a query MagicStack/asyncpg#530)
• Reuse a single DB connection (with transaction block) for all GraphQL queries and mutations performed within a single API request
• Explicitly wrap insert/update/delete queries with transaction blocks (async with conn.begin():) since now the default reset-on-return behavior is "rollback"
• Rename dbpool to db

[codecov]

Codecov Report

Merging #406 (63515e9) into main (b929ba5) will increase coverage by 1.96%.
The diff coverage is 11.06%.

@@            Coverage Diff             @@
##             main     #406      +/-   ##
==========================================
+ Coverage   46.70%   48.66%   +1.96%     
==========================================
  Files          51       52       +1     
  Lines        8250     8267      +17     
==========================================
+ Hits         3853     4023     +170     
+ Misses       4397     4244     -153     

Impacted Files	Coverage Δ
src/ai/backend/gateway/events.py	35.96% <0.00%> (ø)
src/ai/backend/gateway/manager.py	38.23% <0.00%> (ø)
src/ai/backend/gateway/types.py	100.00% <ø> (ø)
src/ai/backend/gateway/utils.py	58.50% <0.00%> (+8.50%)	⬆️
src/ai/backend/manager/models/image.py	57.51% <0.00%> (ø)
src/ai/backend/manager/plugin/error_monitor.py	43.75% <0.00%> (ø)
src/ai/backend/manager/models/user.py	31.91% <1.28%> (+0.30%)	⬆️
src/ai/backend/manager/models/keypair.py	48.59% <2.43%> (+0.93%)	⬆️
src/ai/backend/manager/models/group.py	46.03% <5.08%> (+0.48%)	⬆️
src/ai/backend/manager/models/agent.py	50.00% <5.88%> (+1.40%)	⬆️
... and 27 more

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