Replace CLOBBER_CACHE_ALWAYS with run-time GUC

Forced cache invalidation (CLOBBER_CACHE_ALWAYS) has been impractical
to use for testing in PostgreSQL because it's so slow and because it's
toggled on/off only at build time.  It is helpful when hunting bugs in
any code that uses the sycache/relcache because causes cache
invalidations to be injected whenever it would be possible for an
invalidation to occur, whether or not one was really pending.

Address this by providing run-time control over cache clobber
behaviour using the new debug_invalidate_system_caches_always GUC.
Support is not compiled in at all unless assertions are enabled or
CLOBBER_CACHE_ENABLED is explicitly defined at compile time.  It
defaults to 0 if compiled in, so it has negligible effect on assert
build performance by default.

When support is compiled in, test code can now set
debug_invalidate_system_caches_always=1 locally to a backend to test
specific queries, functions, extensions, etc.  Or tests can toggle it
globally for a specific test case while retaining normal performance
during test setup and teardown.

For backwards compatibility with existing test harnesses and scripts,
debug_invalidate_system_caches_always defaults to 1 if
CLOBBER_CACHE_ALWAYS is defined, and to 3 if CLOBBER_CACHE_RECURSIVE
is defined.

CLOBBER_CACHE_ENABLED is now visible in pg_config_manual.h, as is the
related RECOVER_RELATION_BUILD_MEMORY setting for the relcache.

Author: Craig Ringer <craig.ringer@2ndquadrant.com>
Discussion: https://www.postgresql.org/message-id/flat/CAMsr+YF=+ctXBZj3ywmvKNUjWpxmuTuUKuv-rgbHGX5i5pLstQ@mail.gmail.com

src/backend/utils/adt/lockfuncs.c

@@ -629,7 +629,7 @@ pg_isolation_test_session_is_blocked(PG_FUNCTION_ARGS)
 	 * Check if any of these are in the list of interesting PIDs, that being
 	 * the sessions that the isolation tester is running.  We don't use
 	 * "arrayoverlaps" here, because it would lead to cache lookups and one of
-	 * our goals is to run quickly under CLOBBER_CACHE_ALWAYS.  We expect
+	 * our goals is to run quickly with debug_invalidate_system_caches_always > 0.  We expect
 	 * blocking_pids to be usually empty and otherwise a very small number in
 	 * isolation tester cases, so make that the outer loop of a naive search
 	 * for a match.