1. xact.c: update comments about changing MyProc->xid and MyProc->xmin.

1. xact.c: update comments about changing MyProc->xid and MyProc->xmin.
2. varsup.c:ReadNewTransactionId(): don't read nextXid from disk -
   this func doesn't allocate next xid, so ShmemVariableCache->nextXid
   may be used (but GetNewTransactionId() must be called first).
3. vacuum.c: change elog(ERROR, "Child item....") to elog(NOTICE) -
   this is not ERROR, proper handling is just not implemented, yet.
4. s_lock.c: increase S_MAX_BUSY by 2 times.
5. shmem.c:GetSnapshotData(): have to call ReadNewTransactionId()
   _after_ SpinAcquire(ShmemIndexLock).