Rename create_instance()

This allows for explicit calling the function to create an
instance instead of having to run dlopen.

Change-Id: I79910e29825e976b7fa889a43283a06c6f1f40bb
Reviewed-on: http://review.couchbase.org/41189
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>