Add anti-grinding defense before accessing sealed keys

[amiller]

The initial version of Sirrah had the nice feature that you had to check in on chain before accessing any sensitive information. Of course this is brittle since now the whole network can crash if nodes go offline. Now we used sealed files instead.

The issue is that since a node can access the sealed file entirely offline, after a public disclosure of a vulnerability, unpatched nodes would be able to try running an exploit to get at the sealed data.

A good compromise is the following: still use sealed files, but go through an on-chain check-in process before the node will access it. Now we don't rely on enclave nodes to be up, just the plaintext blockchain. But attackers tipped off by a public disclosure would be limited to exploiting already-running processes, which is harder.