[hammer] check for issuers

[phbnf]

The hammer could check that issuers are set properly.

[AlCutter]

Could you just run the fsck tool after the hammer (or perhaps integrate it with the hammer so it checks as it grows)?

[phbnf]

I think integrating fsck with the hammer would be better if we can, and would also allow implementing some of transparency-dev/tessera#512.