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.
Comment was deleted...just replaying a summary:

Is it OK to use a direct equality comparison here with float/double operands?  Should we introduce an epsilon/delta into the comparison?  If so update tests to touch around this value.