[YSQL] Create index on table with existing data will raise error

[robertpang]

When creating an index on a table with existing data, an error may be raised:

postgres=# CREATE TABLE index_test (col1 int, col2 int);
CREATE TABLE
postgres=# INSERT INTO index_test (col1, col2) VALUES (1, 100);
INSERT 0 1
postgres=# INSERT INTO index_test (col1, col2) VALUES (2, 200);
INSERT 0 1
postgres=# CREATE INDEX index_test_idx1 ON index_test(col1);
ERROR:  Illegal state: Only non-transactional ops should be buffered

The error leaves the session in an inconsistent buffer state and subsequent commands will hang.

[robertpang]

Resolved in commit 0341b29.