fetcher: Clear all data for session in session thread

fetcher: Clear all data for session in session thread

Conceptually the session thread owns the session, so let's clear out
everything predictably there, rather than sometimes having it happen
on the main thread.

Also, this moves up clearing the pending/outstanding queues *before*
we unreference the session, since conceptually they need to reference
it as well.

Based on a patch from: Matthew Barnes <mbarnes@redhat.com>

Closes: #383
Approved by: mbarnes