This patch attempts to avoid excessive allocations and copies when
constructing counter cells using counter_cell_builder. That involves
adding serializer interface to atomic_cell so that the counter cell can
be directly serialized to the buffer allocated for atomic cell.
counter_cell_builder::from_single_shard() is added as well to avoid
std::vector<> overhead when creating a counter cell from a single shard.
The leader receives counter updates as deltas which have to be
transformed to counter shards. In order to do that, current local shard
of the modified counter cell needs to be read, logical clock incremented
and the value modified by the specified delta.
Live counter cells are collections of shards, each one representing the
sum of all operations performed by a particular replica. This commits
introduces an in-memory representation of counters as well as basic
operations such as merge, difference and hashing.
