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.
Add a `@return` comment?