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.
@nmittler - I know we have been using the "global" and "stream specific" terminology in our discussions (and I think this was legacy or I may have added it :) ) but lets try to use specification terminology where appropriate.  [Section 6.9.2](https://tools.ietf.org/html/draft-ietf-httpbis-http2-16#section-6.9.2) is labeled `Initial Flow Control Window Size` should we use this terminology?