Drop Connection classes (or at least make them non-public)

[dhermes]

The Logging / Pub/Sub / Datastore packages all have custom API objects that do that actual communication with the server. Now Connection is just a container for some members that those API objects require. Should we remove these base classes and make any shared behavior from the core Connection into helper functions instead of a shared base?

[dhermes]

Just chatted with @tseaver about this and we both seem to agree, the answer is "yes" to "Should we...".

Sending a PR for BigQuery, Datastore, Logging, Pub/Sub and Storage ASAP.

[theacodes]

With the switch to google-auth, there is even less code in the Connection class - more fuel for this fire.