When a DeltaReplicatedData returns None from delta it must still be
treated as a delta that increase the version counter in DeltaPropagationSelector.
Otherwise a later delta might be applied before the full state gossip is received
and thereby violating RequiresCausalDeliveryOfDeltas.
* Follow up on the causal delivery of deltas.
* The first implementation used full state for the direct
Write messages, i.e. updates with WriteConsistency != LocalWrite
* This is an optimization so that delatas are tried first and if
they can't be applied it falls back to full state.
* For simultanious updates the messages may be reordered because we
create separate WriteAggregator actor and such, but normally they
will be sent in order so the deltas will typically be received in
order, otherwise we fall back to retrying with full state in the
second round in the WriteAggregator.
* keep track of delta interval versions and skip deltas
that are not consequtive, i.e. when some delta message was lost
* send the delta versions in the full state gossip to sync up the
expected versions after dropped deltas
* implementation of deltas for ORSet
* refactoring of the delta types to allow for different type for the
delta and the full state
* extensive tests
* mima filter
* performance optimizations
* simple pruning of deltas
* Java API
* update documentation
* KeyId type alias
* Use InternalApi annotation
* delta GCounter and PNCounter
* first stab at delta propagation protocol
* send delta in the direct write
* possibility to turn off delta propagation
* tests
* protobuf serializer for DeltaPropagation
* documentation
