Refactor native btree updater wrapper to make it reusable

Refactored native btree updater helper code to be
reusable by incremental updater as well as view compactor
delta log apply methods.

During a test run (on Mac/SSD) with 7M documents and 3 simple views,
the taken taken before and after integration of native sequential sorting
and btree updater is as follows:

Before:
Indexing time: 657.884 seconds
Sorting + Btree updates: 603.450 seconds

After:
Indexing time: 386.529 seconds
Sorting + Btree updates: 346.332 seconds

Change-Id: I23172de5fd5a09b8a9f94f37476599f13f41a71e
Reviewed-on: http://review.couchbase.org/32311
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>