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.
Should we add a `ChannelHandlerContext ctx` to this method?  It seems like the current use case of `DefaultHttp2LocalFlowController` is saving the `ctx` from other methods and checking if it is null before use.  Perhaps it will be more clear that this method needs the `ctx` and less boundary conditions if this method is expanded?