Fix/testnet thread debugging fixes

[jcnelson]

Recently, the Argon master node's p2p thread stalled. It did not die -- it still held open the channel linking it to the relayer thread, and it still held the sockets -- but it stopped looping. This PR does the following:

• Sets a panic handler to force the process to abort on thread panic
• Adds better-placed debug messages to the top-level p2p dispatcher, so we can see that the p2p dispatcher is doing something
• Adds better-placed debug messages to the top-level p2p thread and relay thread, so we can see that they are still alive

[lgalabru]

Looks good to me!

[kantai]

LGTM!