We can do CURL requests asynchronously, and we don't need a
synchronous WorkQueue thread for that.

This allows parallelizing lookups and allows immediate cancellation.

memcmp() can overflow the buffer.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Each Upnp*Async() call passes a new cookie pointer, and the cookie
passed to UpnpRegisterClient() appears to be ignored.  Using this
interface is a more elegant approach than having one single "handler"
function.

There will only be very few items, and that does not justify the bloat
of std::map.

Move code to method Start() and add Error& parameters to several
methods.

Forward the Error to the caller.

Reduce bloat.

Memory leak.

Bloat.

Must not perform blocking I/O while holding a mutex.

Remove the ContentDirectoryDescriptor right away.  Reduces bloat.

Eliminate global variables.