[BUGFIX] Check existence of cache files before require

Sometimes an adminstrator clears the cache files in a moment when
another one just tries to require them. Therefore file existence should
be checked before each requirement.

Change-Id: Ia79b8dd7b8943297edfe9e3ac657db1faf151fde
[DB][BUGFIX] Prevent "igbinary_unserialize_string: end-of-data" warning

If you enable the igbinary PHP module for extbase extensions the
t3lib_cache_frontend_VariableFrontend throws a warning. This is because
caching tables don't use blob columns. This patch ALTER content columns
from mediumtext to mediumblob.

Resolves: #28451
Resolves: #56656
Releases: 4.5
Change-Id: I5912ab29328c54933566b3512e3811569aed993b