Feature: List all known keys

[albsch]

Draft implementation of listing all keys in an AntidoteDB data center (over all shards in the cluster). The new API method return a list of bound objects ([{Key, Type Bucket}]).

The riak_core part is implemented and the API in antidote.erl is extended.

To do:

• Check semantics of snapshot time and implemented non ignore Snapshot filter
• Verify again which log entires to use to list keys