Use atomics instead of locking

[nhynes]

In rpc/server.go

func (srv *Web3Gateway) Close() error {
  srv.startStopLock.Lock()
  defer srv.startStopLock.Unlock()

  srv.lock.Lock()
  state := srv.state
  srv.lock.Unlock()

implements an atomic int using a heavy mutex. Using the appropriate atomic datatype would be less surprising. This isn't in a performance critical section of code, so it's not a big issue. It's just a little linty.

But really, why is this one variable atomic? Shouldn't the entire block be protected by a mutex if there's larger global state modification going on?