first pass at support for filter handlers

When a request is created, a new FilterHandler is registered on the event bus.
A FilterHandler wraps a Filter object. The operation response is sent back on
the event bus after the filter has been applied to all of the rows.

This commit adds filter support to SliceHandler. I will add support in GetHandler
in my next commit. Then I need to see about minimizing some of the json object
creation/mapping overhead.
refactoring some boiler plate code for handling reads

This commit also re-enabled another filter test in RawJsonITest.
post-merge clean up
Merge pull request #132 from jsanda/filter-handler

Support for filter handlers