job-queue-delay

we ran into an issue when the number of metadata records was ~3000. the job queue
throttle that is built into the extension was exceeded. there were > 1000 media
file jobs in the queue and while the metadata job attempted to run 5 times the
media file jobs were still > 1000 and thus it failed to add its media file jobs
to the queue.

this change attempts to fix this issue by adding a delay via the jobReleaseTimestamp
param to the metadata job if there are > 1000 media file jobs in the queue. the current
delay is set to Config::$metadata_job_delay = '5 minutes'.

Change-Id: Iad99aca2251b09930c14017cabcc222ebdcd1e42