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

src/include/replication/logical.h

@@ -149,5 +149,7 @@ extern void ResetLogicalStreamingState(void);
 extern void UpdateDecodingStats(LogicalDecodingContext *ctx);
 
 extern bool LogicalReplicationSlotHasPendingWal(XLogRecPtr end_of_wal);
+extern XLogRecPtr LogicalSlotAdvanceAndCheckSnapState(XLogRecPtr moveto,
+													  bool *found_consistent_snapshot);
 
 #endif

src/include/replication/snapbuild.h

@@ -91,4 +91,6 @@ extern void SnapBuildProcessRunningXacts(SnapBuild *builder, XLogRecPtr lsn,
 										 struct xl_running_xacts *running);
 extern void SnapBuildSerializationPoint(SnapBuild *builder, XLogRecPtr lsn);
 
+extern bool SnapBuildSnapshotExists(XLogRecPtr lsn);
+
 #endif							/* SNAPBUILD_H */