Ported reconnect API

[griff]

This is more or less a straight port of the reconnect API from capnp-c++.

I say more or less because I also needed the setTarget functionality from CapRedirector in Sandstorm and so have included that as part of the API.

To keep the API consistent with the existing new_client and new_promise_client functions the auto_reconnect and lazy_auto_reconnect functions work with generic clients that implement FromClientHook.

[dwrensha]

Thanks!