Data channel messages slicing

[vvmnnnkv]

This PR implements data chunking, issue #78
Data is sent as ArrayBuffer, with small header containing message id, number of chunks and chunk number. On receiver side, chunks are stitched together.

There're number of constants that control sending process:

export const WEBRTC_DATACHANNEL_CHUNK_SIZE = 64 * 1024;
export const WEBRTC_DATACHANNEL_MAX_BUFFER = 4 * 1024 * 1024;
export const WEBRTC_DATACHANNEL_BUFFER_TIMEOUT = 2000;
export const WEBRTC_DATACHANNEL_MAX_BUFFER_TIMEOUTS = 5;

Outgoing buffer is where data is buffered after channel.send() but before it is actually sent. When buffer fills up to WEBRTC_DATACHANNEL_MAX_BUFFER, we stop sending and wait for lowbuffer event from the data channel. If event doesn't occur within WEBRTC_DATACHANNEL_BUFFER_TIMEOUT, we re-try checking current buffer size. If the buffer is still larger than WEBRTC_DATACHANNEL_MAX_BUFFER after checking for WEBRTC_DATACHANNEL_MAX_BUFFER_TIMEOUTS times, we consider this to be send error.

I've tested sending 150MB between FF and Chrome located on different remote machines connected via public internet.

[codecov]

Codecov Report

Merging #82 into master will not change coverage.
The diff coverage is 90.9%.

@@           Coverage Diff           @@
##           master      #82   +/-   ##
=======================================
  Coverage   77.16%   77.16%           
=======================================
  Files          20       20           
  Lines         635      635           
  Branches       73       73           
=======================================
  Hits          490      490           
  Misses        116      116           
  Partials       29       29

Impacted Files	Coverage Δ
src/_constants.js	100% <ø> (ø)	⬆️
src/data_channel_message_queue.js	100% <ø> (ø)
src/data_channel_message.js	100% <100%> (ø)
src/webrtc.js	85.33% <75%> (ø)	⬆️

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[cereallarceny]

This is perfect. I only request that you look at changing the class names to avoid conflicts or confusion.

[cereallarceny]

👍