A P-connection drop failed every download for its user, including ones
actively transferring on their own healthy F connection:
collect_failed_tokens now only fails queued downloads. And an F
connection could look its token up before the client-operations thread
had registered it: the listener now waits for the registration, and
claims it only for the matching peer and only when not already claimed.
Fixes#42
Michel de Graaf
f9becf9680