[!!!][FEATURE] Make cache backends more flexible
This change makes cache backends more flexible. It adds a new interface
"TaggableBackendInterface" and a new "SimpleFileBackend". The latter
does not support tagging but is faster in certain cases (where with a
lot of tags/entries flushing everything is faster than flushing by tag).
If you have implemented a cache backend and support tagging, you must
add the "TaggableBackendInterface" to the list of interfaces implemented
by your backend.
In addition this change makes the PDO backend a little more robust and
resolves some code style issues.