fetcher: Move the SoupSession to a separate thread

fetcher: Move the SoupSession to a separate thread

Move the SoupSession to a separate thread with its own isolated main
context and main loop.  All interaction with the SoupSession occurs
by way of idle sources attached to the session's main context, which
execute on the session's thread.

This should solve the problem of running an asynchronous fetch request
synchronously by pushing a new thread-default main context and iterating
a main loop until the request completes.  Prior to this, the new thread-
default main context would interfere with the SoupSession's own async
processing.