Improve the session with Astarte

[joshuachp]

There are some improvements we can make when connecting with Astarte, to make sure we use the least amount of traffic:

• Keep track of the current introspection across restarts
  So only if it was changed (OTA update) we need to make a full connection to Astarte.
• Check the status of the store, if it's missing (probably a new start, manual deletion or factory
  reset) we need to do the full handshake
• Keep track of the completed handshake with Astarte (introspection sent, empty ...), with the full
  handshake completed and session-present true we don't need to send additional data

On a first connection or when we receive a session-present false, we still need to do the full handshake:

• introspection
• empty cache
• purge device properties
• send device properties