pgsql: Improve LISTEN startup time when there are many unread notificat

Improve LISTEN startup time when there are many unread notifications.

If some existing listener is far behind, incoming new listener sessions
would start from that session's read pointer and then need to advance over
many already-committed notification messages, which they have no interest
in.  This was expensive in itself and also thrashed the pg_notify SLRU
buffers a lot more than necessary.  We can improve matters considerably
in typical scenarios, without much added cost, by starting from the
furthest-ahead read pointer, not the furthest-behind one.  We do have to
consider only sessions in our own database when doing this, which requires
an extra field in the data structure, but that's a pretty small cost.

Back-patch to 9.0 where the current LISTEN/NOTIFY logic was introduced.

Matt Newell, slightly adjusted by me

Branch
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/aad86c518d7b3b97c14872258c02551b443536f0

Modified Files
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src/backend/commands/async.c |   50 +++++++++++++++++++++++++++++++++++++-----
1 file changed, 45 insertions(+), 5 deletions(-)