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.
The case when both cells are dead was not handled properly, the diff
was always empty, whereas the cell with higher timestamp should win.
Caused test_mutation_diff_with_random_generator to fail.
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.
