[WIP!!!][BUGFIX] Constant FLUIDCONTENT_TEMPFILE is not always defined

Constant FLUIDCONTENT_TEMPFILE is defined in ext_tables.php which is meant to
point to a file to cache Page TSConfig.

However, this leads to a problem as of TYPO3 6.1 which has a refactored 
caching mechanism. After the global TYPO3 cache has been set (processing of
ext_tables.php and ext_localconf.php among other), the constants is never
defined anymore. As a result Fluid Content is not able to cache Page TSConfig
anymore impacting the overall performance.