New feature: Support for lazy property caching

Lazy caching of properties might be better for performance and user experience than the current default of eager caching.

Currently, every default instantiation of many interfaces such as AsyncDBusProxy will go fetch cached properties. This adds a needless round-trip to the server if the interface is only being used as a one-shot proxy. I expect this type of use to be common - Proxy::destination_unique_name uses it today.

Because it can be useful to report errors in GetAll during the creation of a Proxy, this probably needs to change caching to a tristate (off, on, lazy). If #178 (closed) is resolved by invalidating the cache, the lazy state is basically already required.