security: Support certificate revocation

[bdarnell]

We do not currently support any way to revoke a certificate before it expires, so a compromised certificate could be abused for a long time. We need to support some way for administrators to revoke a compromised certificate.

The best way to do this is probably to store a CRL in a system table, use gossip to notify all nodes whenever it changes, and have each node mirror the current CRL to its local data directory so the last known CRL will be available on restart.

Note that Go's TLS implementation does not support automatic revocation checking, but I think it gives us the tools to check against a CRL by hand (in the x509 package).

[mberhault]

part of #1674, although may be post 1.0.