Defer restoration of libraries in parallel workers.

Defer restoration of libraries in parallel workers.

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