Fix confusion on the padding of GIDs in on commit and abort records.

Fix confusion on the padding of GIDs in on commit and abort records.

Review of commit 1eb6d652: It's pointless to add padding to the GID fields,
when the code that follows assumes that there is no alignment, and uses
memcpy(). Remove the pointless padding.

Update comments to note the new fields in the WAL records.

Reviewed-by: Michael Paquier
Discussion: https://www.postgresql.org/message-id/33b787bf-dc20-1161-54e9-3f3b607bf59d%40iki.fi