TSIG support

[bal-e]

While Cascade was initially intended to support TSIG from the get-go, this feature got lost during the alpha development process. A lot of the infrastructure necessary for it is already in the codebase, it just has to be tied back together. A major concern is going to be the zone loader -- with the current architecture, it's hard to going to be difficult to integrate the new TSIG key store with it. We may have to wait until a new zone loader (#15) is available. TSIG also concerns outgoing zone transfers, for which some form of access control policy needs to be designed.

[Philip-NLnetLabs]

Note that keyset also need TSIG support (for key roll automation). It is best if keyset can access the TSIG secret store in Cascade to avoid duplicating secrets.

[bal-e]

I agree, but that's going to be tricky to implement. The ideal case would be that keyset becomes a library and can directly access Cascade's in-memory TSIG key store. For now, the key manager could verify that the TSIG key store has been synchronized to disk before invoking keyset.