MB-12067 [3/6]: Unit test for degragmenter task

Basic smoketest for the defragmenter: Create a number of documents,
spanning at least two or more pages, then delete most (but not all) of
them - crucially ensure that one document from each page is still
present. This will result in the rest of that page being
'wasted'. Then verify that after defragmentation the actual memory
usage drops down to (close to) mem_used.

Change-Id: Ibfe8780fb02ad5eb504242b01e7583bc1e305184
Reviewed-on: http://review.couchbase.org/41496
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Dave Rigby <daver@couchbase.com>