Fix initialization of RelationSyncEntry for streaming transactions.

Fix initialization of RelationSyncEntry for streaming transactions.

In commit 464824323e, for each RelationSyncEntry we maintained the list
of xids (streamed_txns) for which we have already sent the schema. This
helps us to track when to send the schema to the downstream node for
replication of streaming transactions. Before this list got initialized,
we were processing invalidation messages which access this list and led
to an assertion failure.

In passing, clean up the nearby code:

* Initialize the list of xids with NIL instead of NULL which is our usual
coding practice.
* Remove the MemoryContext switch for creating a RelationSyncEntry in dynahash.

Diagnosed-by: Amit Kapila and Tom Lane
Author: Amit Kapila
Reviewed-by: Tom Lane and Dilip Kumar
Discussion: https://postgr.es/m/904373.1600033123@sss.pgh.pa.us