[YSQL] Allow read committed isolation to pick read time using safe time on first rpc to tserver

[pkj415]

Jira Link: DB-5248

Description

In Read Committed isolation, a new read time is picked for each statement (i.e., a new logical snapshot of the database is used for each statement's reads). This is done (in PgClientService) by setting the read time to the current time at the start of each new statement before issuing requests to any tserver. However, this might results in high latencies in the first read op that is executed as part of that statement because the tablet serving the read (likely on another node) might have to wait for the "safe" time to reach the picked read time. A long wait for safe time is usually seen when there are concurrent writes to the tablet and the read enters while the raft replication that moves the safe time ahead is still in progress (see #11805).

This issue is avoided in Repeatable Read isolation because there, the first tablet serving the read in a transaction is allowed to pick the read time as the latest available "safe" time without having to wait for any catchup. This read time is sent back to PgClientService as used_read_time so that future reads can use the same read time. Note that even in Repeatable Read isolation, in case, there are multiple parallel RPCs to various tservers, the read time is still picked on the PgClientService because otherwise, the rpcs would have to wait for one of them to execute and came back with a used_read_time.