fix(BRIDGE-632): add lock to snapMsgList idx map - #496

Let's have a nice wrapper around this so we never forget to unlock things (not just here; but unify the usage)

func (l *snapMsgList) withLock(fn func ()) {
   l.lock.Lock()
   defer l.lock.Unlock()
    fn ()
}

isn't this overcomplicating it, there are 5 calls to list.idx

I'd enforce the convention to always use that instead of bare lock calls. + better DRY

Yeah but we don't want the whole function to be locked in all cases + only Read lock in some

• there would be a need to create example updateUnsafe + update which update just calls withLock(updateUnsafe(...)), which is a lot of extra code for again 5 calls to this map