Exception handling during job reservation
Revert "Parallelize stop and restart to be faster with many workers."

Daemons has major issues dealing with threaded daemonizing
This reverts commit 7757209bb5a85de9ab1c8a5eec4894678c11366e.
Merge remote-tracking branch 'origin/pr/537'
Prevent a worker from entering an infinite backend error loop
Provide a hook for a DJ backend to attempt recovery from a reservation error

An example usage would be for DJ active record to reconnect if mysql "goes away"
Prepare 4.0.0 release

[ci-skip]
Fix delayed_job_command