The cache files in typo3conf/temp_CACHED_* were by default set to a hash that
was based on the extensionlist, but this is quite unnecessary, so the extCache
option was removed to be only used once in the typo3conf/ directory in order
to prevent hundreds of temp_CACHED_* files that don't get cleaned up
automatically. From now on the option $TYPO3_CONF_VARS['EXT']['extCache'] can
only be set to 0 or 1.

Change-Id: Ic6081deeb306d8a7ef8b5f3d358c1d63ddb74ae8
Resolves: #28040
Reviewed-on: http://review.typo3.org/3197
Reviewed-by: Georg Ringer
Reviewed-by: Andreas Wolf
Tested-by: Andreas Wolf
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog