(#635) - move more logic to generic adapter (auto_compaction)

In order auto_compaction to work I moved modification of the whole
rev_tree into compactDocument in generic adapter. It then sends
revisions which we want to delete and modified rev_tree to adapter
specific method which does the compaction process.
(#635) - merge_rev_tree tests now support availability

That's quite to much said as they don't check if the availability
is merged correctly.