Add an additional ClientContext interface that can propagate context
Add an additional ClientContext interface that can propagate context

This propagates context to HTTP requests, which means client deadlines
will correctly propagate (and other items that use context if setup).

Implemented by adding a new interface to keep the existing client
interface the same (maybe overly paranoid, but avoids breaking any tests
mocking interfaces).

bluebreezecf/opentsdb-goclient,bluebreezecf/opentsdb-goclient