Protect client registration session (regc) with mutex

[bennylp]

The original idea of not using mutex in regc was because operations should be serialized by 8-bit boolean flags, and (I heard) operation to 8-bit variables should be atomic. But it seems that with recent modifications/bug fixes in regc, this rule may not hold anymore (such as the timer, which I think was added only quite recently).

So I guess adding a mutex will not hurt, and it even should add robustness to the code.