The communication between the ShardRequestManager and the RsmClient
used to be direct. In this PR this changes into a future-based
communication type. The RsmClient stores state about the currently
processed future (either read or write request) and exposes blocking
and non-blocking functionality to obtain the filled future. The
ShardRequestManager -for now- will send of the set of requests present
in the ExecutionState and block on each of them until the requests are
completed or the set of paginated responses(caused by, for example the
batch-limit in ScanAll) are ready for the next round.
