SoupSession: allow creating a "plain" SoupSession for use with new APIs

SoupSession: allow creating a "plain" SoupSession for use with new APIs

In gio-based APIs, async vs sync is a function-level distinction, not
a class-level distinction. Merge most of the existing SoupSessionAsync
and SoupSessionSync code up into SoupSession, and make SoupSession
non-abstract, so that you can create a SoupSession and then use either
sync or async SoupRequest-based APIs on it. (The traditional APIs
still require one of the traditional subclasses, although the code
reorg does affect them in some ways, such as making SoupSessionAsync
more thread-safe.)