This commit fixes the bug 3583921, datanode/coordinator slave crash.

This was caused by incorrect handling of KnownAssignedXids for XC,
where there could be missing Xids in WAL.   Now because XC does not
support consistent database view throught hot standby, KnownAssignedXids
are simply disabled.

Correct handling will be implemented when XC supports consistent
database view through hot standby.

	modified:   src/backend/storage/ipc/procarray.c