Avoid accumulating relation locks during sequence synchronization.

Avoid accumulating relation locks during sequence synchronization.

While collecting the sequences to synchronize, the sequence sync worker
opened each INIT sequence with RowExclusiveLock and held it until the
transaction committed. With many such sequences, this could exhaust the
shared lock table and fail with "out of shared memory".

The worker only reads each sequence's identity (namespace and name) here
and needs it to stay stable while read, for which AccessShareLock is
enough, as it conflicts with the AccessExclusiveLock taken by DROP,
RENAME, and SET SCHEMA. Take that lock instead and release it as soon as
the identity is read. The later synchronization re-opens each sequence, so
it does not rely on the lock being retained.

Reported-by: Noah Misch <noah@leadboat.com>
Author: vignesh C <vignesh21@gmail.com>
Reviewed-by: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Backpatch-through: 19, where it was introduced
Discussion: https://postgr.es/m/20260710045217.f0.noahmisch@microsoft.com