[lib] handle Tunnelbroker messages sequentially

Summary: This is similar approach to what's implemented on web worker: https://phab.comm.dev/D11299

Test Plan: Connect two devices, send message trough Tunnelbroker and add sleep to processing, make sure each messages are processed in sequence.

Reviewers: tomek, bartek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11732