[YSQL] Use session flag to set transactional writes for index inserts

[emhna]

Description

Today, backfill_write_time is used to determine whether the distributed transactional write should be used which is only set during index backfills.
In order to disable the distributed transactional writes for regular inserts to index, we will use the yb_disable_transactional_writes session flag.
Using this flag will help improve the latency performance of bulk data loading for index tables such as when COPY command is used which goes into the insert write path (not delete or update).