Minor threading fixes and improvements

[ppebb]

Contains 3 fixes/improvements for threading

• Config is properly cached and locked behind a mutex as it is accessed from multiple routines at once.
• qr2 and natneg use nhttp.AtomicBool to trigger shutdown, as normal Go bools are not atomic. I doubt this impacts any legitimate behavior, but it is more correct.
• Better handling of the rpcMutex in main, primarily not accessing it without a lock on shutdown.

In addition, the state folder used for serializing connections is never generated by wwfc, so I've made that happen on backend launch. Unrelated to any other changes, I just didn't want to make another PR. If desired, I can remove the commit from this branch and create a separate PR.

[cetio]

Your work in Operation Desert Storm was phenomenal, but I think this is the most beautiful thing I have ever seen.
LGTM

[chairclr]

Loved your work in Iraq! LGTM