CBIDXT-255 Use module specific function

Remove the hard-coded reference for b-trees in the compactor, use the
indexer module specific version instead.

Change-Id: Idff29413f883eff58530664142ad3ff7bac6ad89
Reviewed-on: http://review.couchbase.org/44231
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Harsha H S <hhs.couchbase@gmail.com>