[TASK] Make MemcachedBackend a transient backend

Allows passing non-string values to the backend, which
is perfectly allowed for this type of backend. The change
means that VariableFrontends used with this backend will
store non-string values without serializing, thus optimising
performance and transparency.

Key changes:

* Exceptions are no longer thrown when a non-string is passed
* Chunk-splitting does not happen on non-strings; entries
   exceeding the maximum bucket size get logged and ignored.
* Serializer decision is delegated to memcached configuration.

Change-Id: Ie11736be621a2dd27bfde60b82cd5f6b3a04d981
Resolves: #80246
Releases: master
Reviewed-on: https://review.typo3.org/52015


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Claus Due <claus@phpmind.net>
Tested-by: Claus Due <claus@phpmind.net>
Reviewed-by: Elmar Hinz <t3elmar@gmail.com>
Tested-by: Elmar Hinz <t3elmar@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>