only allow a single poll call at once
only allow a single poll call at once

Otherwise, if the server times out (mongo issue, etc), we can have many poll
calls piling up and making the server stop accepting requests.

This has a nice benefit of throttling polling for participants on a
clogged local wifi network.

pol-is/polisClientParticipation,pol-is/polisClientParticipation,pol-is/polisClientParticipation,pol-is/polisClientParticipation,pol-is/polisClientParticipation