[PATCH] PostgresNode.pm enhancements, pg_lsn helper, and some more recovery tests

Every test I write with the TAP framework for recovery seems to need
to wait for one node to catch up to another or examine replication
slot state. So: attached is a patch to add helper methods for these
tasks.

Also add a new pg_lsn.pm helper class to deal with PostgreSQL's
awkward representation of LSNs in perl. Made extra fun by our use of
Perl 5.8.8 with no new modules, so we can't rely on having 64-bit
integers. Provides sensible LSN comparisons. I'll be using this in
coming patches that have to make assertions about differences between
LSNs, and I think it's sensible to have anyway. Incorporates tests for
the class.

Finally, add some coverage of physical replication slots to recovery tests.

Backpatch to 9.6 desirable, since we seem to be doing that for TAP
infrastructure.

These three are related enough, and all only touch the TAP framework,
so I've bundled them in a series.

--
Craig Ringer http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services