add an optional cache

[ThomasWaldmann]

If the data storage is not local (= for all backends except posixfs), borgstore could use an optional cache and use a backend (like posixfs) internally for cache storage.

Cache coherency is very important:

• it is quite easy if objects are named by their content hash: only presence vs. non-presence of object with a specified name. due to name == hash(content), the content of some named object will never change, but could be deleted.
• for arbitrarily named objects: don't cache at all in first implementation. this should be no big problem, because this is usually small stuff, like a config or other metadata.

Cache eviction:

• in first implementation, do not evict anything from the cache
• later, a LRU based eviction could be used, separately configurable per namespace

Notes:

• the levels configuration of the cache and the server-side storage backend would be identical
• objects in the cache would have identical names as in the remote backend
• if the cache has unlimited size, it can provide a complete local mirror of the remote backend
• cache is only updated after we get a 200 OK response for the respective request to the server
• we have to expect multiple concurrent users updating the server-side backend independently
• list method could be uncached for simplicity

[ThomasWaldmann]

Check where it is best to implement this:

• in the rest backend (useful only when using that)
• in Store (so it would be useful for most backends)