tests: mvcc: Encapsulate memory management details
tests: mvcc: Encapsulate memory management details

Curently tests have a single LSA region lock around construction of
managed objects, their manipulation, and access. This way we avoid the
complexity of dealing with allocating sections. That will not be
possible once apply_to_incomplete() is changed to enter an allocating
section itself becasue this requires region to be unlocked at
entry. The tests will have to take more fine-grained locks. That is
somewhat tricky add would add a lot of noise to tests. This patch will
make things easier by abstracting LSA management, among other things,
inside mvcc_conatiner and mvcc_partition classes.

avikivity/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla