pgsql: Optimize LISTEN/NOTIFY via shared channel map and direct advance

Optimize LISTEN/NOTIFY via shared channel map and direct advancement.

This patch reworks LISTEN/NOTIFY to avoid waking backends that have
no need to process the notification messages we just sent.

The primary change is to create a shared hash table that tracks
which processes are listening to which channels (where a "channel" is
defined by a database OID and channel name). This allows a notifying
process to accurately determine which listeners are interested,
replacing the previous weak approximation that listeners in other
databases couldn't be interested.

Secondly, if a listener is known not to be interested and is
currently stopped at the old queue head, we avoid waking it at all
and just directly advance its queue pointer past the notifications
we inserted.

These changes permit very significant improvements (integer multiples)
in NOTIFY throughput, as well as a noticeable reduction in latency,
when there are many listeners but only a few are interested in any
specific message. There is no improvement for the simplest case where
every listener reads every message, but any loss seems below the noise
level.

Author: Joel Jacobson <joel(at)compiler(dot)org>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/6899c044-4a82-49be-8117-e6f669765f7e@app.fastmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/282b1cde9dedf456ecf02eb27caf086023a7bb71

Modified Files
--------------
src/backend/commands/async.c | 1141 +++++++++++++++++++----
src/backend/utils/activity/wait_event_names.txt | 1 +
src/include/storage/lwlocklist.h | 1 +
src/test/isolation/expected/async-notify.out | 124 ++-
src/test/isolation/specs/async-notify.spec | 72 ++
src/tools/pgindent/typedefs.list | 6 +
6 files changed, 1161 insertions(+), 184 deletions(-)