Disable slow-start on TCP sockets

Leaving Nagle's algorithm enabled on the TCP transport leads to some
really long latencies (at least, during the first several messages).

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=75544
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
NEWS for dbus-1.8
Merge branch 'dbus-1.8'
NEWS for 1.9
.gitignore: ignore /test-driver, generated by recent Automake