Add a job timeout configuration to time out jobs that have run too long

[bensheldon]

Timeouts in Ruby are rather dangerous so this should be given some thought as it's probably unavoidable to entirely avoid the risk of corruption of shared state.

Concurrent Ruby has Cancellations but it seems impossible to safely cancel a uncontrolled code (e.g. active job): http://ruby-concurrency.github.io/concurrent-ruby/master/Concurrent/Cancellation.html

I think the biggest danger around an ActiveJob is corrupting the ActiveRecord database pool. So adding a timeout solely around the ActiveJob code, but within the executor wrap might work. Even better would be running it within an ActiveJob around hook so that it could potentially be caught by ActiveJob's rescue_from/retry_on handlers.

[bensheldon]

Killing threads is not something actually supported by Concurrent Ruby because of the difficulty in doing it safely with user code. Added instructions for configuring timeouts in user code: #39