protocol-level wait-for-LSN

This is something I hacked together on the way back from pgconf.eu.
It's highly experimental.

The idea is to do the equivalent of pg_wal_replay_wait() on the protocol
level, so that it is ideally fully transparent to the application code.
The application just issues queries, and they might be serviced by a
primary or a standby, but there is always a correct ordering of reads
after writes.

Additionally, I'm exploring whether this is an idea for a protocol
extension that might be a bit more complex than, say, longer cancel
keys, something we could have a discussion around protocol versioning
around.

The patch adds a protocol extension called _pq_.wait_for_lsn as well as
a libpq connection option wait_for_lsn to activate the same. (Use e.g.,
psql -d 'wait_for_lsn=1'.)

With this protocol extension, two things are changed:

- The ReadyForQuery message sends back the current LSN.

- The Query message sends an LSN to wait for. (This doesn't handle the
extended query protocol yet.)

To make any real use of this, you'd need some middleware, like a hacked
pgbouncer, that transparently redirects queries among primaries and
standbys, which doesn't exist yet. But if it did, I imagine it could be
pretty useful.

There might be other ways to slice this. Instead of using a
hypothetical middleware, the application would use two connections, one
for writing, one for reading, and the LSN would be communicated between
the two. I imagine in this case, at least the one half of the protocol,
shipping the current LSN with ReadyForQuery, could be useful, instead of
requiring application code to issue pg_current_wal_insert_lsn() explicitly.

Thoughts?