Track mine processes, fixes #5729

This pull request adds support for scheduled jobs being added to the /var/cache/salt/minion/procs directory so we can keep track of them and prevent too many of the same process from being started at the same time.  It also includes a small performance optimization for listing processes--in one case in the code we don't need to grab a list of all the processes and check against each one--we only need to know if the process we care about is still running.
@thatch : you probably want to look at this somewhat closely.

This fixes #5729.