[YSQL] Support READ COMMITTED isolation level

[pkj415]

Jira Link: DB-3140

Description

Requirements for GA -

• [YSQL] Support READ COMMITTED isolation level semantics for DMLs #9468 (present from v2.15.0.0, v2.14.0.0, v2.13.0, v2.12.2.0)
• [YSQL] Match Pg semantics for volatile functions in READ COMMITTED isolation level #11640 (present from v2.15.1.0, v2.14.1.0)
• [YSQL] READ COMMITTED semantics should apply to all non-DDL statements (not just INSERT, UPDATE, DELETE, and SELECT) #12254 (present from v2.15.1.0, v2.14.1.0)
• [YSQL] Integrate YSQL follower reads feature with READ COMMITTED isolation level #11641 (present from v2.15.1.0, v2.14.0.0, and expected in next v2.12.* release)
• [YSQL] Disable lazy evaluation in functions in READ COMMITTED isolation level #13428 (expected in next 2.15.* release and all future major versions)
• [YSQL] Allow read committed isolation to pick read time using safe time on first rpc to tserver #15856
• [YSQL] Allow client to set isolation level after "begin;" when yb_enable_read_committed_isolation=true #12494 (this might be solved automatically once #11572 is fixed (i.e., if we are able to get rid of the savepoint based infrastructure for READ COMMITTED isolation after #11572 is done, we wouldn't hit this issue)
• [YSQL] Track statement retries per statement type in pg_stat_statements #16718

Limitations (in order of priority) -

• [YSQL] Integrate READ COMMITTED isolation with wait queue based Wait-on-Conflict concurrency control to detect deadlocks and for higher performance #13211
• [YSQL] Ensure no kConflict errors are thrown in a READ COMMITTED txn even if statement's output exceeds ysql_output_buffer_size (gflag with default of 256KB). #11572 (this would be solved for kConflicts automatically once #11573 is fixed)
• [YSQL] Non-transactional side-effects can occur more than once when a conflict/ read restart error in functions/ procedures is handled by read committed isolation level #12958 (this would be solved for kConflicts automatically once #11573 is fixed. It might also be solved for KReadRestarts once #11572 is fixed)

Open items/ Good to haves

• [YSQL] Avoid rpc from YSQL to local tserver when switching sub transaction #14202
• [YSQL] Handle conflicts in READ COMMITTED by performing Pg style “READ COMMITTED Update Checking” #11573
• [YSQL] Enable lazy evaluation in functions in READ COMMITTED isolation level #12959

Open bugs

• [YSQL] List of aborted sub-txns is removed by asynchronous heartbeats #13222 (present from v2.14.1.0, expected in next 2.15.* release)