Refactoring HTTP/2 Flow Control interfaces.

Motivation:

The terminology used with inbound/outbound is a little confusing since
it's not discussed in the spec. We should switch to using local/remote
instead. Also there is some asymmetry between the inbound/outbound
interfaces which could probably be cleaned up.

Modifications:

Changing the interface names and making a common Http2FlowController
interface for most of the methods.

Result:

The HTTP/2 flow control interfaces should be more clear.
Agree on a name change.  Given that the other method is `sendFlowControlledFrame` it seems that `lastFlowControlledFrameSent` is reasonable.  I'm not sure we need to clarify beyond that since the spec is very clear about DATA being the only flow controlled frames. If we were to get more specific, I would suggest renaming the other method as well, but I think it would start to get needlessly verbose.  If you're ok, I think I'll stick with `lastFlowControlledFrameSent` for now.