Refactor the processing of the view group btree information

De-clutter the code a bit and move the processing of the btree
specific parsing into its own function.

Change-Id: Iaba910e94dd1b923dd7e4c8b5c4e981bd4d1ad93
Reviewed-on: http://review.couchbase.org/39628
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>