Make it less generic, to avoid clashes.

Closes GH-479.

Closes #477

Destruct automatically, even if leaving the scope due to exception
being thrown.

The list of partitions is cleared automatically.

This replaces the old `dead` flag which was unreliable; it was `false`
if the EventThread was not yet started, which could cause deadlocks in
BlockingCall().

They will be stopped by ~EventThread() when the `Instance` is deleted.