1. MyProc->xid assignment is moved to GetNewTransactionId so newer

   transactions will not assume that MyProc transaction was committed
   before snapshot calculations. With old MyProc->xid assignment
   (in xact.c:StartTransaction()) there was ability to see the same
   row twice (I used gdb for this)!...
2. Assignments of InvalidTransactionId to MyProc->xid and MyProc->xmin
   are moved from xact.c:CommitTransaction() to
   xact.c:RecordTransactionCommit() - this invalidation must be done
   before releasing transaction locks or bad (too high) XmaxRecent value
   might be used by vacuum ("ERROR:  Child itemid marked as unused"
   reported by "Hiroshi Inoue" <Inoue@tpf.co.jp>; once again, gdb
   allowed me reproduce this error).