Up to now, async.c scanned its whole array of per-backend state
whenever it needed to find listening backends.  That's expensive
if MaxBackends is large, so extend the data structure with list
links that thread the active entries together.

A downside of this change is that asyncQueueUnregister (unregister
a listening backend at backend exit) now requires exclusive not shared
lock, and it can take awhile if there are many other listening
backends.  We could improve the latter issue by using a doubly- not
singly-linked list, but it's probably not worth the storage space;
typical usage patterns for LISTEN/NOTIFY have fairly long-lived
listeners.

In return for that, Exec_ListenPreCommit (initially register a
listening backend), SignalBackends, and asyncQueueAdvanceTail
get significantly faster when MaxBackends is much larger than
the number of listening backends.  If most of the potential
backend slots are listening, we don't win, but that's a case
where the actual interprocess-signal overhead is going to swamp
these considerations anyway.

Martijn van Oosterhout, hacked a bit more by me

Discussion: https://postgr.es/m/CADWG95vtRBFDdrx1JdT1_9nhOFw48KaeTev6F_LtDQAFVpSPhA@mail.gmail.com