Rename btree_op functions

The btree_op functions can also be used for the spatial views, hence
rename them to a name, not referring to "btree".

This is a pure refactoring without any functional change.

Change-Id: I8e20189e2cd733711d76935f6d012c1f5908f0d8
Reviewed-on: http://review.couchbase.org/44232
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Harsha H S <hhs.couchbase@gmail.com>