CXX-295 Allow count to work with query hints

[TylerBrock]

@acmorrow, it seems we can take advantage of the fact that Query::hint() had a useless overload that took a string. What I did was repurpose that method to take an index name as opposed to a json string representation of the index spec. This way we can use Query to hold a hint that can either be either an index spec (BSONObj) or index name (std::string).

This is good because:

• When the query gets unpacked in the _countCmd method we can use either to support current and future server versions.
• The api isn't terrible because we avoid adding another parameter to the count method.
• It fits well with our current design.

[acmorrow]

LGTM, nice to see we didn't need to rip apart Query. Seems like there is some very slight risk for older users of the API you repurposed, but seems pretty unlikely.