pgsql: Replace CLOBBER_CACHE_ALWAYS with run-time GUC

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/4656e3d66893f286767285cf74dabb3877068e49

Modified Files
--------------
doc/src/sgml/config.sgml            | 37 ++++++++++++++++++++
doc/src/sgml/regress.sgml           |  3 +-
src/backend/utils/adt/lockfuncs.c   |  2 +-
src/backend/utils/cache/inval.c     | 48 +++++++++++++-------------
src/backend/utils/cache/plancache.c |  2 +-
src/backend/utils/cache/relcache.c  | 67 ++++++++++++++++++++++---------------
src/backend/utils/misc/guc.c        | 24 +++++++++++++
src/include/pg_config_manual.h      | 39 +++++++++++++++++++++
src/include/utils/inval.h           |  1 +
9 files changed, 169 insertions(+), 54 deletions(-)