race condition in sequencer rotation

[mtsitrin]

When handling a rotation signal, there's a race condition between closing the errgroup and writing on the rotateC channel:

// we get here once a sequencer rotation signal is received
m.logger.Info("Sequencer rotation started.", "next_seq", nextSeqAddr)
go func() {
  rotateC <- nextSeqAddr
}()

And:

go func() {
  _ = eg.Wait()
  // Check if exited due to sequencer rotation signal
  select {
    case nextSeqAddr := <-rotateSequencerC:
	    m.handleRotationReq(ctx, nextSeqAddr)
    default:
	    m.logger.Info("Block manager err group finished.")
    }
}()