You can now pass a dictionary of request headers when starting a new
download in ProtocolServer.
The HTTP and HTTPS protocol will include the headers in their requests.
Previously a download lived independently of the client connection it came
from. This was the source of several undesirable behaviours, including the
potential for clients to influence downloads they didn't start, and
downloads living longer than their associated client connections. Now we
attach downloads to client connections, which means they're cleaned up
automatically when the client goes away, and there's significantly less
risk of clients interfering with each other.
These are supposed to be interpreted caselessly so let's just use the
case insensitive traits throughout. This means we'll understand things
like "Content-Length" even when they send "content-length" etc.
It didn't feel right to have a "DHCPClient" in a "Servers" directory.
Rename this to Services to better reflect the type of programs we'll
be putting in there.
