[Yamux] Send whole data if window size is > 0

[StefanBratanov]

There were some inconsistencies with sendFrames method and the send buffer flush approach to sending data.

• sendFrames was checking if window size is <= 0 and adding to buffer if so and afterwards was slicing the data based on the window size and sending across. So essentially if window size is 10 and data is 100 bytes, it will send across 10 frames of 10 bytes.
• flush was respecting the window size and was sending partial data if the length of the data is less than the window size.

I would imagine a better approach is if window size is > 0, just send the whole data respecting the maxFrameDataLength.

• Made flush not use sendFrames since the data is already limited by maxFrameDataLength so no slicing needed
• Renamed sendFrames to sendData
• Changed sendData to respect the window size and add to buffer there instead of onChildWrite
• Modified tests accordingly
• Release message when onChildWrite and no window size is found.

[Nashatyrev]

LGTM