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serve: Ensure redis-server saves after signal

This is part one of improved shutdown handling; part two will come in a
future commit involving a more graceful shutdown of the main node
process itself.

I began to notice that running `./go serve` to launch the redis-server,
using the app, and then killing the script quickly resulted in data
loss. This didn't happen when running redis-server manually, as a signal
sent to that process would result in it saving its data before exiting.
The problem appeared to be that signals sent to the `./go serve` script
would be sent simultaneously to the redis-server process, under which
circumstances it wouldn't save its data automatically.

The script now launches redis-server in a separate process group so that
any signals sent to the script aren't sent to the redis-server process.
The custom-links server itself now runs as a background process so that
the it can be managed explicitly via a trap on the TERM, INT, and HUP
signals. Finally, an EXIT trap sends the `shutdown save` command to the
redis-server and waits for the process to exit to complete a graceful
shutdown with all data saved.

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