Implement split and stitch for large data channel messages in iOS

[cereallarceny]

This was already done in syft.js: https://github.com/OpenMined/syft.js/issues/78

This will also need to be done for iOS. You should read these two classes so that you fully understand the problem and theoretical solution:

• https://github.com/OpenMined/syft.js/blob/master/src/data_channel_message.js
• https://github.com/OpenMined/syft.js/blob/master/src/data_channel_message_queue.js

[mjjimenez]

Data channel message

• [ ] Construct chunk header
• [ ] Divide message into chunks with appropriate chunk header
• [ ] Add individual data chunks according to chunk index
• [ ] Reassemble chunks when all of them are complete

Data channel queue

• [ ] Store all data channel messages being received
• [ ] Inform observers when a message is completed

RTC Connection buffer

• [ ] Check first if it's possible to monitor peer connection message buffer in iOS