For clarity, and to make merging easier.

The code to manage the hash table of "pending resync EOFs" for append-only
tables is moved to smgr_ao.c. One notable change here is that the
pendingDeletesPerformed flag is removed. It was used to track whether there
are any pending deletes, or any pending AO table resyncs, but we might as
well check the pending delete list and the pending syncs hash table
directly, it's hardly any slower than checking a separate boolean.

There are still plenty of GPDB changes in smgr.c, but this is a good step
forward.