Changes for calculateQuantityJob config option

Using 0.8.6

Actual behavior:

calculateQuantityJob exists as a config option to determine how often the inventory snapshot table's contents are refreshed. The original default was once a day at midnight, but now that the calculation is faster, it can be increased to once every five minutes or even once a minute. However, this option is not documented anywhere.

Expected behavior:

• Documentation about why changes aren't always immediately visible should be written/updated/bolded/put at the very top of the front page of the manual
• calculateQuantityJob should be included in the example config in the documentation
• calculateQuantityJob should have its meaning and format explained so users can specify their desired frequency easily (not everyone knows what a crontab is)
• calculateQuantityJob should have a more sane default of every 5 minutes
• The actual job for calculateQuantityJob should have locking to prevent multiple instances from running at the same time, so that even if the user enters too frequent a value for the system to keep up, it will simply update as fast as it's able

Steps to reproduce:

1. Go to docs.openboxes.com and search for calculateQuantityJob.
2. Note the lack of results.
3. Go to the config file section of the docs and read through all of the options.
4. Don't find calculateQuantityJob anywhere.

See also: #1016

Search the repository and the config documentation for calculateQuantityJob, then inspect the job entry point and its current default. Compare the implementation with the requested documentation, five-minute default, and locking behavior; the work is done when the option is documented clearly and the scheduling behavior prevents overlapping runs.