Worker threads for packet encryption

[amishshah]

After a very rudimentary test, I found that 40 seconds of playback of an Ogg Opus audio stream had spent 105ms time blocking the thread with packet encryption using sodium on my system.

This could contribute to stream stutters at scale.

Offloading this task to worker threads can help to reduce this.

[amishshah]

This issue has been deprioritised in favour of #53 for the time being.

[amishshah]

This issue is being closed as, after some rough prototyping, it isn't worth the extra overhead and random breakages that seem to occur.