Several users of extensions complained of crashes in parallel workers
that turned out to be due to syscache access from their _PG_init()
functions.  Reorder the initialization of parallel workers so that
libraries are restored after the caches are initialized, and inside a
transaction.

This was reported in bug #15350 and elsewhere.  We don't consider it
to be a bug: extensions shouldn't do that, because then they can't be
used in shared_preload_libraries.  However, it's a fairly obscure
hazard and these extensions worked in practice before parallel query
came along.  So let's make it work.  Later commits might add a warning
message and eventually an error.

Back-patch to 9.6, where parallel query landed.

Author: Thomas Munro
Reviewed-by: Amit Kapila
Reported-by: Kieran McCusker, Jimmy
Discussion: https://postgr.es/m/153512195228.1489.8545997741965926448%40wrigleys.postgresql.org