Useful dst_key functions

Useful dst_key functions

Add a couple of dst_key functions for determining hints that
consider key states if they are available.
- dst_key_is_unused:
  A key has no timing metadata set other than Created.
- dst_key_is_published:
  A key has publish timing metadata <= now, DNSKEY state in
  RUMOURED or OMNIPRESENT.
- dst_key_is_active:
  A key has active timing metadata <= now, RRSIG state in
  RUMOURED or OMNIPRESENT.
- dst_key_is_signing:
  KSK is_signing and is_active means different things than
  for a ZSK. A ZSK is active means it is also signing, but
  a KSK always signs its DNSKEY RRset but is considered
  active if its DS is present (rumoured or omnipresent).
- dst_key_is_revoked:
  A key has revoke timing metadata <= now.
- dst_key_is_removed:
  A key has delete timing metadata <= now, DNSKEY state in
  UNRETENTIVE or HIDDEN.