pgsql: Ensure that the sync slots reach a consistent state after promot

Ensure that the sync slots reach a consistent state after promotion without losing data.

We were directly copying the LSN locations while syncing the slots on the
standby. Now, it is possible that at some particular restart_lsn there are
some running xacts, which means if we start reading the WAL from that
location after promotion, we won't reach a consistent snapshot state at
that point. However, on the primary, we would have already been in a
consistent snapshot state at that restart_lsn so we would have just
serialized the existing snapshot.

To avoid this problem we will use the advance_slot functionality unless
the snapshot already exists at the synced restart_lsn location. This will
help us to ensure that snapbuilder/slot statuses are updated properly
without generating any changes. Note that the synced slot will remain as
RS_TEMPORARY till the decoding from corresponding restart_lsn can reach a
consistent snapshot state after which they will be marked as
RS_PERSISTENT.

Per buildfarm

Author: Hou Zhijie
Reviewed-by: Bertrand Drouvot, Shveta Malik, Bharath Rupireddy, Amit Kapila
Discussion: https://postgr.es/m/OS0PR01MB5716B3942AE49F3F725ACA92943B2@OS0PR01MB5716.jpnprd01.prod.outlook.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2ec005b4e29740f0d36e6646d149af192328b2ff

Modified Files
--------------
src/backend/replication/logical/logical.c | 147 ++++++++++++++++++++-
src/backend/replication/logical/slotsync.c | 133 +++++++++++++------
src/backend/replication/logical/snapbuild.c | 23 ++++
src/backend/replication/slotfuncs.c | 118 +----------------
src/include/replication/logical.h | 2 +
src/include/replication/snapbuild.h | 2 +
.../recovery/t/040_standby_failover_slots_sync.pl | 97 +++++++++++---
7 files changed, 351 insertions(+), 171 deletions(-)