[PGPRO-4074] 3pc support.

Cherry pick of cef39e64b85aed8cd (persistent state change for prepared xact) plus
1) Allow setting state_3pc in PREPARE;
2) Variable len state_3pc -- it can be quite large once we stuff the whole
   generational stuff inside;
3) Fix holes in state_3pc interactions with checkpointer:
   - 'P, ckpt, 3pc, ckpt, crash' previously lost 3pc as gxact is marked ondisk
     during second ckpt;
   - prepare_start|end_lsn wasn't updated during state_3pc redo, this also led
     to state_3pc loss once the xact is moved to file;
   - gxact's start|end lsns shouldn't be updated until
     SetPreparedTransactionState fsyncs its WAL;
4) Relevant TAP test moved into core patch and supplemented with 3) cases.
5) Bump TWOPHASE_MAGIC.

tags: multimaster
(cherry picked from commit d45aa55aebb60540416559ad9c929a118056bcce)

Author: arssher

src/backend/access/rmgrdesc/xactdesc.c

@@ -230,10 +230,19 @@ ParsePrepareRecord(uint8 info, xl_xact_prepare *xlrec, xl_xact_parsed_prepare *p
 	parsed->nrels = xlrec->ncommitrels;
 	parsed->nabortrels = xlrec->nabortrels;
 	parsed->nmsgs = xlrec->ninvalmsgs;
+	parsed->state_3pc_change = xlrec->state_3pc_change;
 
 	strncpy(parsed->twophase_gid, bufptr, xlrec->gidlen);
 	bufptr += MAXALIGN(xlrec->gidlen);
 
+	if (xlrec->state_3pc_len > 0)
+	{
+		strncpy(parsed->state_3pc, bufptr, xlrec->state_3pc_len);
+		bufptr += MAXALIGN(xlrec->state_3pc_len);
+	}
+	else
+		*parsed->state_3pc = '\0';
+
 	parsed->subxacts = (TransactionId *) bufptr;
 	bufptr += MAXALIGN(xlrec->nsubxacts * sizeof(TransactionId));