lib: Don’t close the server socket immediately when stopping the server

If we do, it’s possible for the final iteration of the server’s main
loop to process an incoming packet and to reference a NULL socket
pointer. Instead, use the more conventional soup_server_quit() call, and
allow the socket to be closed when the server is destroyed, which should
always be shortly afterwards because UhmServer completely controls the
lifetime of the SoupServer.

https://bugzilla.gnome.org/show_bug.cgi?id=731040
lib: Make SoupServer usage thread safe

SoupServer is not inherently thread safe, so we must marshal all calls
to it through the main context of its worker thread.

https://bugzilla.gnome.org/show_bug.cgi?id=731040
build: Remove pkg-config file on distclean rather than clean

It’s generated by configure, not by make, so should not be removed on
clean.
build: Hard-code API version in documentation build

Unfortunately gtk-doc can’t cope with the variable substitution.
build: Remove outdated documentation checks

These are now performed by gtkdoc-check. Yay!
Release version 0.3.0
build: Increment version to 0.4.0