Make pg_controldata report newest XID with valid commit timestamp

Make pg_controldata report newest XID with valid commit timestamp

Previously pg_controldata didn't report newestCommitTs and this was
an oversight in commit 73c986a.

Also this patch changes pg_resetxlog so that it uses the same sentences
as pg_controldata does, regarding oldestCommitTs and newestCommitTs,
for the sake of consistency.

Back-patch to 9.5 where track_commit_timestamp was added.

Euler Taveira