Fix Ghost execution bug and multiple execution bug

http://www.concrete5.org/developers/bugs/5-6-2-1/ghost-execution-of-queuable-jobs/
http://www.concrete5.org/developers/bugs/5-6-2-1/slices-of-many-queable-jobs-could-be-executed-together/
Merge pull request #1516 from JohntheFish/patch-15

Fix Ghost execution bug and multiple execution bug