Fix postmaster's behavior during smart shutdown.

 Fix postmaster's behavior during smart shutdown.
 
 Up to now, upon receipt of a SIGTERM ("smart shutdown" command), the
 postmaster has immediately killed all "optional" background processes,
 and subsequently refused to launch new ones while it's waiting for
 foreground client processes to exit.  No doubt this seemed like an OK
 policy at some point; but it's a pretty bad one now, because it makes
 for a seriously degraded environment for the remaining clients:
 
 * Parallel queries are killed, and new ones fail to launch. (And our
 parallel-query infrastructure utterly fails to deal with the case
 in a reasonable way --- it just hangs waiting for workers that are
 not going to arrive.  There is more work needed in that area IMO.)
 
 * Autovacuum ceases to function.  We can tolerate that for awhile,
 but if bulk-update queries continue to run in the surviving client
 sessions, there's eventually going to be a mess.  In the worst case
 the system could reach a forced shutdown to prevent XID wraparound.
 
 * The bgwriter and walwriter are also stopped immediately, likely
 resulting in performance degradation.
 
 Hence, let's rearrange things so that the only immediate change in
 behavior is refusing to let in new normal connections.  Once the last
 normal connection is gone, shut everything down as though we'd received
 a "fast" shutdown.  To implement this, remove the PM_WAIT_BACKUP and
 PM_WAIT_READONLY states, instead staying in PM_RUN or PM_HOT_STANDBY
 while normal connections remain.  A subsidiary state variable tracks
 whether or not we're letting in new connections in those states.
 
 This also allows having just one copy of the logic for killing child
 processes in smart and fast shutdown modes.  I moved that logic into
 PostmasterStateMachine() by inventing a new state PM_STOP_BACKENDS.
 
 Back-patch to 9.6 where parallel query was added.  In principle
 this'd be a good idea in 9.5 as well, but the risk/reward ratio
 is not as good there, since lack of autovacuum is not a problem
 during typical uses of smart shutdown.
 
 Per report from Bharath Rupireddy.
 
 Patch by me, reviewed by Thomas Munro
 
 Discussion: https://postgr.es/m/CALj2ACXAZ5vKxT9P7P89D87i3MDO9bfS+_bjMHgnWJs8uwUOOw@mail.gmail.com