lib: Make SoupServer usage thread safe
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

MathieuDuponchelle/uhttpmock,MathieuDuponchelle/uhttpmock,pwithnall/uhttpmock,pwithnall/uhttpmock,MathieuDuponchelle/uhttpmock,Distrotech/uhttpmock,Distrotech/uhttpmock