Create an Unattach rpc so that dying clients can reattach

When my server kept running and my clients failed, a new client was unable to reattach because it was trying to use the same fid.

A client that dies should be able to clean up after itself.

There's a few other problems with Attach:
1. If two clients try to attach at the same time with fid:1 one is out of luck.  Instead we should just choose a fid for them and say we did it.
2. If we choose a fid for them, the serverfid just keeps increasing, we don't look for a hole in the fidMap like we should.