Xapian bridge now supports offset parameter

This adds support for 'offset' parameter in the xapian bridge.
The offset specifies from where in the result set to start
returning results. We now also return totalResults and offset
as part of the response from the xapian bridge.

https://github.com/endlessm/eos-sdk/issues/1802

hmm, we may actually want to rethink how to get the total value. from the docs (http://xapian.org/docs/apidoc/html/classXapian_1_1MSet.html#aeab0a82126d9f2cf0ef5321549ea9775)

`This value is returned because there is sometimes a request to display such information. However, our experience is that presenting this value to users causes them to worry about the large number of results, rather than how useful those at the top of the result set are, and is thus undesirable.`