Caching framework for box

[enkore]

Depends on finalized specification for Qabel/qabel-block/issues/64 ✓

Ze grand image here is that we want to enable push support (and/or more efficient polling) in the clients with as little per-client-not-invented-here problems as possible.

Therefore we need a cache for DMs in the box module, integrated with the various moving parts that need them.

The requirements of q-desktop is that updated DMs are pushed to it, while the android app will stick for polling for now. For the latter use case the cache should be able to check it's contents against the block server in an efficient way, ie. using HTTP pipelining. The latter part should be abstract (I feel that due to the usual issues with Apache HC on android this part is best implemented in q-android).

[enkore]

Updated docker has block with push support now.

Added implementation hints and samples to Qabel/qabel-block#64 which also has the protocol spec for now.