Auto shutdown and restart bot periodically

[tk3369]

Resolves #50

Here's the current design:

• start_bot function now takes a kwarg run_duration
• The new auto_shutdown function spawns an async task to keep track of time
• When current time exceeds start_time + run_duration then it just exits program
• The run.sh script has a loop that runs forever, unless RUN_ONCE environment variable is set to yes
• The run.sh script also accepts an environment variable RUN_DURATION_MINUTES which is set to 6 hours by default

Note that this is a simple design and not super-robust. For example, if there's a transaction happening (e.g. writing to a JSON file for some command) and the program exits abruptly then we may have a corrupted file. Given the current traffic, I don't think this is a big issue at the moment. We can fix that later if the bot is used more heavily.