This patch changes the implementation of atomic_cell and
atomic_cell_or_collection to use the data::cell implementation which is
based on the new in-memory representation infrastructure.
As a prepratation for the switch to the new cell representation this
patch changes the type returned by atomic_cell_view::value() to one that
requires explicit linearisation of the cell value. Even though the value
is still implicitly linearised (and only when managed by the LSA) the
new interface is the same as the target one so that no more changes to
its users will be needed.
Due to a bug in an implementation of UUID less compare some Scylla
versions sort counter shards in an incorrect order. Moreover, when
dealing with imported correct data the inconsistencies in ordering
caused some counter shards to become duplicated.
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.
