SENSEI-325 Allow MultiBoboBrowser and BoboSubBrowser to be created by pl...

...ugins in Sensei

This can be used to implement features like custom DF and other interesting features without having to reimplement Lucene Queries.

Requires Bobo change to add close() to Browsable interface:
https://senseidb.atlassian.net/browse/BOBO-288
https://github.com/senseidb/bobo/pull/17

https://senseidb.atlassian.net/browse/SENSEI-325