Return early from send_c_find after doing send_c_cancel and/or releasing association

[moloney]

Is your feature request related to a problem? Please describe.
If a user starts a C-FIND, then changes their mind and sends a C-CANCEL (and/or closes the association) it would be nice if the send_c_find method would return in a timely fashion. Currently it appears to block the thread until some network timeouts are reached.

Describe the solution you'd like
Have some means of avoiding this long timeout on an operation we know will not succeed.