Handle accepted task handoff for verifiers

[jules]

While testing for #252, I noticed that my test failed due to the pending task not having any way of being reassigned, as there were no verifiers left. This doesn't seem to be a problem in any of the other timeout tests, which seems like a bug to me - those tasks still should be handed off, but they apparently just kind of disappear.

The drop_participant logic should be updated so that all verifier tasks are handed off properly.

[kellpossible]

More generally we should test what happens when verifiers are dropped from the ceremony at least in the integration tests, and ensure that it doesn't stall

[kellpossible]

This line should be investigated more: #288 (comment) perhaps as part of this issue.