Allow to clear only some caches in the composite driver

[gggeek]

When using a stacked cache systrem, it is useful to be able to clear some caches while keeping the others intact, f.e. clearing the memory cache while keeping the disk cache full is a common thing for long-running cli scripts

[jhallbachner]

This might better be raised as an issue on the underlying Stash library, since the best place to implement passthrough functionality to individual drivers would be directly in the Composite driver itself.