poll: be more resilient to restarts

poll: be more resilient to restarts

When restarting or reconnecting to servers, the channel objects may
change, causing odd crashes when counting votes during poll closure.

This is easily fixed by letting poll.channel hold a string rather than
an object, but past polls reloaded from the registry will already have
actual Channel object stored, so we need to convert them. Do this, and
keep track of whether it has been done or not.