Improve handling of reseted hosts

[daonb]

**What's missing? **
When I'm reconnecting to a server that's just been restarted , I get a message
"data channel open but no first message" and nothing happens.

It happens because Terminal7 stores the server's old state from before the
reset and tries to re-open old panes.
To solve the issue we need to add a session hash to the state and
ensure it's as expect.
On first connection, when the state is empty Terminal7 will randomly generate
a hash and add it to the state under the session key.

When Terminal7 reconnects to a host it will first get the state using the
get_payload API endpoint to ensure the session hasn't changed.
If it did (or if there's no session key) Terminal7 will clear his state,
start using the one from the server and notify the user: "fresh webexec session".

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