Fix xid_advance_interval when max_retention_duration is 0.

Fix xid_advance_interval when max_retention_duration is 0.

When a subscription has retain_dead_tuples enabled and maxretention is
zero (unlimited), adjust_xid_advance_interval() mistakenly caps
xid_advance_interval to zero.

This zero interval forces get_candidate_xid() to evaluate
TimestampDifferenceExceeds() as always true, causing the apply worker to
call GetOldestActiveTransactionId() for every WAL message. This
leads to unnecessary ProcArrayLock acquisitions.

Fix this by only capping the interval when maxretention > 0, allowing
the exponential back-off to function properly.

Author: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>
Reviewed-by: shveta malik <shveta.malik@gmail.com>
Reviewed-by: Nisha Moond <nisha.moond412@gmail.com>
Discussion: https://postgr.es/m/CAHg+QDdKVnCLHot=AcoPpEiSyDzGz7wGYjAFHVOw57oDtmUDWQ@mail.gmail.com