map mail-id to cc-info and persist that data

[azul]

In order to retrieve a block for an old mail we need to be able to look it up without relying on having the entire mail.
muacrypt keeps a log of all mails it sees. But it does not store all the headers of the encrypted part. Therefore it also does not store the GossipClaims and ChainStore headers. It does however store the mail-id. So if we remember the GossipClaim and ChainStore values for a mail-id we can retrieve them later on.

[azul]

not sure we need this anymore. It's more an optimization. We could as well just use the latest block from every chain and follow the chain from there on.