Summary:
The correctness bug happens because of not idempotent modification of
the template request in PgGate. Parallel query executes the PgGate
statement multiple times, and only first execution produces correct
request to the DocDB, subsequent executions may lose scan keys and
return non-matching rows. This fix disables the modifications if
request boundaries are set.

Additional fix is the DocDB's presumption that full-key requests do not
have bounds. Therefore DocDB ignored bounds on a request with full key.

That is a suboptimal quick fix. It may cause performance regression in
some corner cases. We plan for follow-up fix to address those
performance issues.
Jira: DB-12405

Test Plan: ./yb_build.sh release --java-test 'org.yb.pgsql.TestPgRegressParallel#testPgRegressBigParallel'

Reviewers: dmitry, tnayak

Reviewed By: dmitry

Subscribers: ybase, yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D39947