Fix race conditions in synchronous standby management.

 Fix race conditions in synchronous standby management.
 
 We have repeatedly seen the buildfarm reach the Assert(false) in
 SyncRepGetSyncStandbysPriority.  This apparently is due to failing to
 consider the possibility that the sync_standby_priority values in
 shared memory might be inconsistent; but they will be whenever only
 some of the walsenders have updated their values after a change in
 the synchronous_standby_names setting.  That function is vastly too
 complex for what it does, anyway, so rewriting it seems better than
 trying to apply a band-aid fix.
 
 Furthermore, the API of SyncRepGetSyncStandbys is broken by design:
 it returns a list of WalSnd array indexes, but there is nothing
 guaranteeing that the contents of the WalSnd array remain stable.
 Thus, if some walsender exits and then a new walsender process
 takes over that WalSnd array slot, a caller might make use of
 WAL position data that it should not, potentially leading to
 incorrect decisions about whether to release transactions that
 are waiting for synchronous commit.
 
 To fix, replace SyncRepGetSyncStandbys with a new function
 SyncRepGetCandidateStandbys that copies all the required data
 from shared memory while holding the relevant mutexes.  If the
 associated walsender process then exits, this data is still safe to
 make release decisions with, since we know that that much WAL *was*
 sent to a valid standby server.  This incidentally means that we no
 longer need to treat sync_standby_priority as protected by the
 SyncRepLock rather than the per-walsender mutex.
 
 SyncRepGetSyncStandbys is no longer used by the core code, so remove
 it entirely in HEAD.  However, it seems possible that external code is
 relying on that function, so do not remove it from the back branches.
 Instead, just remove the known-incorrect Assert.  When the bug occurs,
 the function will return a too-short list, which callers should treat
 as meaning there are not enough sync standbys, which seems like a
 reasonably safe fallback until the inconsistent state is resolved.
 Moreover it's bug-compatible with what has been happening in non-assert
 builds.  We cannot do anything about the walsender-replacement race
 condition without an API/ABI break.
 
 The bogus assertion exists back to 9.6, but 9.6 is sufficiently
 different from the later branches that the patch doesn't apply at all.
 I chose to just remove the bogus assertion in 9.6, feeling that the
 probability of a bad outcome from the walsender-replacement race
 condition is too low to justify rewriting the whole patch for 9.6.
 
 Discussion: https://postgr.es/m/21519.1585272409@sss.pgh.pa.us