[DocDB] Use statement start time to compute wait time of waiters in pg_locks

[robertsami]

Jira Link: DB-9250

Description

The query layer tracks a statement start time for each running statement. We have previously used the time at which a waiter enters the wait queue to display a "wait_time" to users of pg_locks.

However, this is unreliable due to the reasons detailed in the following issues:
#17009
#16286
#16389

A more reliable approach would be to use the pg statement start time provided by the query layer in the WriteQueryRPC it sends to docdb

Issue Type

kind/bug

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