Fixes deadlock bug.  Closes #210

This commit also puts an instance of RemoteTagScanner into the
Instance class, and hooks it into the "add" and "addid" commands.

Fixup for commit 681e012f

Works around a deadlock bug which happens when waiting for an
EventLoop which isn't running anymore.

Fixes another part of #208, the one which caused the bogus exception.

Fixes deadlock due to mutex double lock.

Closes #208.

Allows more forward declarations.

To be used when a URL gets added.

This change was missing in 47fa8c4c

The scan methods must be "noexcept".

The CurlRequest constructor can throw exceptions, therefore we must
not use "noexcept".