[ccache] Using ccache with memcached

[Anders Björklund]

> The option to switch the to_cache/from_cache can be made available
> separately, like it was in your PR. But it can use another config ?
> Probably needs some updating and refactoring, and it would be nice
> to try and keep the code duplication between them to a minimum...
>
> i.e. between the current filesystem code and the memcached code
>
> I can make an attempt to merge them, or if you want to do it...
> To add a config like "memcached_only", next to "memcached_conf" ?
> If you have a single server, then *neither* option makes any sense.
> So it all depends on the setup, and needs to benchmarked further...

Here is such an attempt, to keep *both* features available:
https://github.com/itensionanders/ccache/tree/memcached-only

If you set memcached_only to true, it will not use the fs cache.
Currently it will store the manifests locally, as in the original.

Also added a basic unit test to it.

Needs some cleanup, but works OK ?

/Anders