Add more options for xmins and nextxids

[addisonj]

This adds two more options for additional metadata into the slot:

• include-xmins This data includes the catalog_xmin and xmin fields
  from the replication slot. The reasons for wanting this generally occurs when
  using logical replication with a secondary. Since replication slots don't
  get replicated to secondaries, in the event of the failover you need
  some mechanism for seeing what records you may have missed. The
  xmin/catalog_xmin is useful as you can use it for a lower bound and any
  records with a higher xmin could have been missed
• include-next-xids This includes sending both the epoch and
  nextxid 32-bit ints. The epoch is a number that increments when xid
  rollover happens. This allows you to reconstruct the same txid you get
  from the txid_current function which accounts for rollover. This can
  be really useful to see at a point's time how far behind the slot you
  currently are.

[eulerto]

@addisonj You submitted two features and it should be two pull requests or at least two commits. I saw that you change some unrelated lines, fix them.
I can see the usefulness of xmin and catalog_xmin while postgres does not support failover slots (replication slots on replica). Tests aren't useful.
I'm not sure someone will use include-next-xids to track replication lag. It is simple to query pg_stat_replication or even pg_replication_slots.

[jmealo]

@addisonj: seems fair to pull apart the PR but I can definitely see the value.