server: add an unblocking lane

[titoBouzout]

if the socket connection is busy uploading or downloading too much data, then that will block small messages
solutions could be

• open two socket connections
• • wasteful but allows easy two-way communication
• use regular request for vast amount of data
• • hard to sync but doable, server pushes to clients by instructing the client to download from an url, client pushes to server by just posting data
• split the big messages
• • as the messages are big, you cannot send or receive data meanwhile you sending these big chunks, so maybe we can split it in smaller chunks and always giving priority to whatever is in queue