This new loop retries individual statements inside an explicit READ
COMMITTED transaction. This is possible because each statement in a READ
COMMITTED transaction has a different read timestamp. The conn executor
already has retry logic for all implicit transactions, and we continue to
use those when possible.

A session setting controls how many retries will be performed for a statement
inside of an explicit READ COMMITTED transaction.

Release note (sql change): Added the
max_retries_for_read_committed session variable. It
defaults to 10, and determines the number of times an individual
statement in an explicit READ COMMITTED transaction will be retried
if it encounters a retriable transaction error.

Release note: None

This test ensures that we do not do per-statement retries for READ
COMMITTED transactions if results were already sent to the client.

Release note: None