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.

Prepare to refactor some code.

Delete the object when closing the database.

Delete the object when closing the database.

The method emplace() was added in gcc 4.8.  This commit restores
compatibility with gcc 4.7.