proposal: get oldest LSN - function

Hello, I want to suggest a client-side little function, implemented
in the attached patch.

Function pg_oldest_xlog_location gets us the oldest LSN (Log Sequence
Number) in xlog.

It is useful additional tool for DBA (we can get replicationSlotMinLSN,
so why not in master), it can show us, if xlog replication or wal-sender
is working properly or indicate if replication on startup can get up to
date with master, or after long turnoff must be recovered from archive.

Anyway, does it look useful enough to be part of postgres?
I guess I should push this to commitfest if that's the case.

Best regards,

--
Ivan Kartyshov
Postgres Professional: www.postgrespro.com
Russian Postgres Company