Add a new shmem_request_hook hook.

Currently, preloaded libraries are expected to request additional
shared memory and LWLocks in _PG_init().  However, it is not unusal
for such requests to depend on MaxBackends, which won't be
initialized at that time.  Such requests could also depend on GUCs
that other modules might change.  This introduces a new hook where
modules can safely use MaxBackends and GUCs to request additional
shared memory and LWLocks.

Furthermore, this change restricts requests for shared memory and
LWLocks to this hook.  Previously, libraries could make requests
until the size of the main shared memory segment was calculated.
Unlike before, we no longer silently ignore requests received at
invalid times.  Instead, we FATAL if someone tries to request
additional shared memory or LWLocks outside of the hook.

Nathan Bossart and Julien Rouhaud

Discussion: https://postgr.es/m/20220412210112.GA2065815%40nathanxps13
Discussion: https://postgr.es/m/Yn2jE/lmDhKtkUdr@paquier.xyz

src/backend/postmaster/postmaster.c

@@ -1042,6 +1042,11 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	InitializeMaxBackends();
 
+	/*
+	 * Give preloaded libraries a chance to request additional shared memory.
+	 */
+	process_shmem_requests();
+
 	/*
 	 * Now that loadable modules have had their chance to request additional
 	 * shared memory, determine the value of any runtime-computed GUCs that