Closes #155

Bugs found by Coverity.

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.

libupnp provides seconds, not whatever time unit is used by
std::chrono::steady_clock.

Necessary for "final" on gcc 4.6.

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.

Pass "const char *" to Expat.

Remove the ContentDirectoryDescriptor right away.  Reduces bloat.

Eliminate global variables.