[BUGFIX] t3libExtMgm does not cache extension version

The t3lib_extMgm::getExtensionVersion function includes
the ext_emconf.php file of an extension each time it is called.

The version is cached using the CF runtime cache.

Change-Id: I05502a035a597b5b2b09e60d4027db47d8e2ac71
Resolves: #28233
Releases: 4.6
Reviewed-on: http://review.typo3.org/3353
Reviewed-by: Georg Ringer
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter