(SERVER-116) Reuse client instances

Reuse HTTP client instances in the ruby code rather than creating
a new one for every request.
What if the caller wants connections to be reused?  I don't know if that's a use-case we need to consider, but it seems weird to just ignore any `Connection` header the caller may have specified here.  I suppose reusing the connection would just be an optimization anyway, but ... still ... feels weird.