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.
It may be helpful to clarify this a bit.  For example is this the "number of bytes remaining in the window", or the "number of bytes used in the window", or something else?  Either way would it be useful to provide methods for both concepts?