Support automatic TA key rollover

[ydahhrk]

At present, TALs are largely static, and because Relying Parties rely on them to find out the public key of a particular Trust Anchor's signature, it's difficult for RIRs to replace their Trust Anchor keys. This is a security liability of the RPKI at large.

There are ongoing efforts to standardize a secure and automatic TAL update mechanism, and FORT should implement it as soon as possible.

The draft is draft-ietf-sidrops-signed-tal.