Have GetCurrentTransactionStopTimestamp() set xactStopTimestamp if unset

Have GetCurrentTransactionStopTimestamp() set xactStopTimestamp if unset

Previously GetCurrentTransactionStopTimestamp() computed a new timestamp
whenever xactStopTimestamp was unset and xactStopTimestamp was only set when a
commit or abort record was written.

An upcoming patch will add additional calls to
GetCurrentTransactionStopTimestamp() from pgstats. To avoid computing
timestamps multiple times, set xactStopTimestamp in
GetCurrentTransactionStopTimestamp() if not already set.

Author: Dave Page <dpage@pgadmin.org>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Vik Fearing <vik@postgresfriends.org>
Discussion: https://postgr.es/m/20220906155325.an3xesq5o3fq36gt%40awork3.anarazel.de