Deprecate search documents and migrate search functionality to tables

[rsinger]

There is functionally no difference between search documents and table documents.

I propose we merge the two concepts into an improved tablespec and migrate the search functionality to MongoTripodTables

[scaleupcto]

👍

[scaleupcto]

I think this was originally done to differentiate the back ends. But it would instead be better to be able to specify different back ends on a per-table basis