Adding window maintenance flag to HTTP/2 inbound flow control

Motivation:

Currently, window maintenance is automatically performed when a flow
control window drops below half its initial size. We should provide a
way for advanced applications to determine whether or not this should be
done on a per-stream basis.

Modifications:

Modifying DefaultHttp2InboundFlowController to allow enabling/disabling
of window maintenance per stream.

Result:

Inbound flow control window maintenance will be dynamically
configurable.
Change `f` constant identifier to `d`?