Force pruning during data migration

Force pruning during data migration

Our current PruningDataTreeModification performs pruning only in
two cases:
1) the root is being written (i.e. Snapshot recovery)
2) DataTree reports a validation issue

This is not sufficient for uint-type migration, as we have to
subject all writes and merges to pruning/translation irrespective
of where they occur.

Split up PruningDataTreeModification into two implementations,
Proactive and Reactive and use them as appropriate based on migration
requirements.

JIRA: CONTROLLER-1923
Change-Id: I4184c56380b7f52a8d7af6f17346f9c5edd31b28
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>