Encountering tombstones while transforming counter update from deltas to
shards is expected to be rare due to the fact that counter cells cannot
be recreated once removed.
This assumption makes it unnecessary to care much about removed cells
during delta->shard transformation as it adds complexity to the code and
is not required to produce correct results.
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.
