`fn handle_events` would panic, if there were multiple events for one
token, and the first one that happened to be handled was an error.
Indeed it is concerning if a token is encountered that was never
registered before. However, tokens that were just deregistered must be
tracked.
Using `Vec` here seems a bit costly, in the future,
`smallvec::SmallVec` could be considered.
The "unregister" methods are renamed to "deregister" to better line up
with `mio` vocabulary.
Log statements that helped analysis of the panic that occurred here
are overhauled and improved, requiring a `Debug` bound on types that
obviously implement it.
0b3424857e