Agent Protocol Updates

[vs49688]

Everything that should be in the next agent protocol update:

• Protocol versioning
  • A singe number field would suffice. Agents are shipped with Nimrod, so this isn't a problem.
• SSH key path field
  • So SSH transfer targets can be used securely
  • Or embed the key directly
• state info in agent.pong
  • for accounting and debugging purposes
• a new "resync" message
  • If agents get out-of-sync for some reason, the master can force-reset them.
• a new CommandResult.Status enum value of Failed
  • Used for explicitly stating a process has returned nonzero. Blocks Attempts aren't failed if the final command fails #16
• add a timestamp field to each message
  • To AMQPBasicProperties (POSIX timestamp)
  • timestamp field (ISO8601)
  • X-NimrodG-Sent-At header (ISO8601)
    • time the message was actually sent
• message signing
  • NIM1-HMAC-SHA224
  • NIM1-HMAC-SHA256
  • NIM1-HMAC-SHA384
  • NIM1-HMAC-SHA512
  • Change datestamp to timestamp
  • Use nonce
  • Add a nonce field. For future use.
• agent.submit token field
  • See Remove NimrodServeAPI #29