fix: deadlock when signing with the same validator in parallel

[y0sher]

Description

Together with @moshe-blox and with some input from @vaclav-ssvlabs and @zktaiga , we realized the change we are doing to alan might try signing with the same validator (for different slots) at the same time in parallel.

Regression test added.

The bug

// get validator lock
Goroutine1 -> mapLock.Lock
Gorotouine1 (defer mapLock.Unlock)
     Goroutine1 -> ValidatorLock.Lock()
defer executed

// second get validator lock
Goroutine2 -> mapLock.Lock()
   Goroutine2 -> ValidatorLock.Lock() // stuck here

// release first validator lock
Goroutine1 -> mapLock.Lock() // stuck here before first is still stuck on ValidatorLock
** DEADLOCK **

[moshe-blox]

LGTM