XactLockTableWait doesn't set wait_event correctly

New (9.4) XactLockTableWait() sets the reason for the wait, so that if
we emit a log message then it will produce a log message to say its
not waiting on a transaction, its waiting on a lock.

Nice. log_lock_waits works great.

New (9.6) wait_event code is not correctly set. In ProcSleep() we set
the wait reason according to the actual lock tag, effectively ignoring
the information specifically provided by XactLockTableWait().

Ugh.

pg_stat_activity shows a wait_event of 'transactionid' rather than 'tuple'
pg_locks shows a transactionid lock rather than a wait for tuple lock

That looks like a bug since we get different answers from
log_lock_wait and wait_event, which is annoying and confusing.

-- 
Simon Riggs                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services