Add DESTROY cell to TunnelCommunity

[egbertbouman]

When a relay drops a circuit, it sends a DESTROY cell to the adjacent nodes on that circuit. Nodes receiving an DESTROY cell will:

• free resources associated with the corresponding circuit
• drop all subsequent cells for that circuit
• forward the DESTROY message to the next hop (unless it's the end of the circuit)

The payload of a DESTROY cell contains the reason why the circuit is being destroyed. Possible reasons are (subset of TOR):

NONE - No reason given.
PROTOCOL - Protocol violation.
INTERNAL - Internal error.
HIBERNATING - Not currently operating; trying to save bandwidth.
RESOURCELIMIT - Out of memory, sockets, or circuit IDs.
CONNECTFAILED - Unable to reach relay.
FINISHED - The circuit has expired because it's inactive, old, or has exceeded its traffic limit.
TIMEOUT - Circuit construction took too long

For now, we only support destroying a circuit completely.

[whirm]

Depends on #1066

[devos50]

@whirm what's the status of this issue?

[whirm]

That's tunnel community stuff, disregard it.

[devos50]

This has been implemented already: 4b098ae

Please reopen this issue if I'm incorrect 👍