Fix WAIT FOR LSN cleanup on subtransaction abort

Fix WAIT FOR LSN cleanup on subtransaction abort

WAIT FOR LSN registers the current backend in shared memory before entering an
interruptible wait loop.  Top-level abort and backend exit already call
WaitLSNCleanup(), but subtransaction abort did not.  If an interrupt, such as
statement_timeout, occurred while waiting inside a savepoint, rolling back to
the savepoint left the backend marked as present in the WAIT FOR LSN heap.

Clean up WAIT FOR LSN state from AbortSubTransaction() as well, and add
a TAP test covering reuse of WAIT FOR LSN after a savepoint rollback.

Reported-by: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Discussion: https://postgr.es/m/CAJTYsWXDRwo-RVRaQgwxVcXgURVFeX8BKnijQrPiPcSCkDDX9A%40mail.gmail.com
Author: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Author: Xuneng Zhou <xunengzhou@gmail.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>