Add some assertions in syncrep.c

Add some assertions in syncrep.c

A couple of routines assume that the LWLock SyncRepLock needs to be
taken, so add a couple of assertions to be sure of that.  Also, when
waiting for a given LSN at transaction commit, the code implied that the
syncrep queue cleanup happens while holding interrupts, but the code
never checked after that.

Author: Michael Paquier
Reviewed-by: Fujii Masao, Kyotaro Horiguchi, Dongming Liu
Discussion: https://postgr.es/m/a0806273-8bbb-43b3-bbe1-c45a58f6ae21.lingce.ldm@alibaba-inc.com