Allow Scheduler to automatically clean up preserved jobs every N jobs or seconds

[bensheldon]

This will be enabled by default in GoodJob 3.0.

The general idea is a garbage collector doing a sweep on a set interval: both number of jobs executed, and a time-based interval. The time-based interval is currently reliant on a job being executed i.e. the trigger for the time-based interval is a job being executed (there is not a separate cron-like process)

Adds two new configuration options:

• cleanup_interval_jobs (integer) Number of jobs a Scheduler will execute before cleaning up preserved jobs. Defaults to nil. Can also be set with the environment variable GOOD_JOB_CLEANUP_INTERVAL_JOBS.
• cleanup_interval_seconds (integer) Number of seconds a Scheduler will wait before cleaning up preserved jobs. Defaults to nil. Can also be set with the environment variable GOOD_JOB_CLEANUP_INTERVAL_SECONDS.

Uses the existing (but awkwardly named) cleanup_preserved_jobs_before_seconds_ago configuration option.

Closes #412