Replaces #1255.

• [ ] authenticate against ID@host:port in the RemoteSigner, check that id matches configured
• [ ] Persist PrivKey the SocketClient uses for external signers to authenticate properly
  • replaced by: privval: authenticate and persist keys for tcp conns #3105
• Bring back LastSignInfo (PrivValidator follow up #1255 (comment))
  • done in R4R: Split immutable and mutable parts of priv_validator.json #2870
• [ ] Address notes/review in privval: set deadline in readMsg #2548
  • nothing really here
• [ ] Integration tests supporting priv_val_server and KMS
  • replaced by privval: integration test for priv_val_server and for KMS #3106
• [ ] Test recovery from connection failure/timeout with the KMS
  • done in Close and retry a RemoteSigner on err #2923