* Datagrams:

  - Redefine the repliable datagram signature for non-DSA_SHA1 sig types;
    was the sig of the SHA-256 of the payload, now the sig of the payload itself.
    This is an incompatible change but nobody is yet using the new
    sig types for datagram applications.
  - Don't pollute the hash cache with hashes of payloads
  - Check for too-big datagrams
  - Remove assertion check
  - Cleanups