Make walsender more responsive.

Per testing by Andres Freund, this improves replication performance
and reduces replication latency and latency jitter.  I was a bit
concerned about moving more work into XLogInsert, but testing seems
to show that it's not a problem in practice.

Along the way, improve comments for WaitLatchOrSocket.

Andres Freund.  Review and stylistic cleanup by me.

src/backend/access/transam/twophase.c

@@ -1042,13 +1042,6 @@ EndPrepare(GlobalTransaction gxact)
 
 	/* If we crash now, we have prepared: WAL replay will fix things */
 
-	/*
-	 * Wake up all walsenders to send WAL up to the PREPARE record immediately
-	 * if replication is enabled
-	 */
-	if (max_wal_senders > 0)
-		WalSndWakeup();
-
 	/* write correct CRC and close file */
 	if ((write(fd, &statefile_crc, sizeof(pg_crc32))) != sizeof(pg_crc32))
 	{
@@ -2045,13 +2038,6 @@ RecordTransactionCommitPrepared(TransactionId xid,
 	/* Flush XLOG to disk */
 	XLogFlush(recptr);
 
-	/*
-	 * Wake up all walsenders to send WAL up to the COMMIT PREPARED record
-	 * immediately if replication is enabled
-	 */
-	if (max_wal_senders > 0)
-		WalSndWakeup();
-
 	/* Mark the transaction committed in pg_clog */
 	TransactionIdCommitTree(xid, nchildren, children);
 
@@ -2132,13 +2118,6 @@ RecordTransactionAbortPrepared(TransactionId xid,
 	/* Always flush, since we're about to remove the 2PC state file */
 	XLogFlush(recptr);
 
-	/*
-	 * Wake up all walsenders to send WAL up to the ABORT PREPARED record
-	 * immediately if replication is enabled
-	 */
-	if (max_wal_senders > 0)
-		WalSndWakeup();
-
 	/*
 	 * Mark the transaction aborted in clog.  This is not absolutely necessary
 	 * but we may as well do it while we are here.