This really only implements a heuristic, assuming that HTTP/1.0 servers
cannot handle having multiple active connections; this assumption has
lots of false positives, but ultimately HTTP/1.0 is an out-of-date HTTP
version and people using it should just switch to a newer standard
anyway.
Specifically, python's "SimpleHTTPRequestHandler" utilises a
single-threaded HTTP/1.0 server, which means no keepalive and more
importantly, hangs and races with more than a single connection present.
This commit makes it so we serialise all requests to servers that are
known to serve only a single request per connection (aka HTTP/1.0 with
our setup, as we unconditionally request keepalive)