Allow configurable refresh times for commit checks

It appears that the DVCS connector is polling for new commits once an hour:

This is far too long an interval (perhaps chosen to avoid DDOS'ing bitbucket?), and is leading developers to think the integration is broken. We use a local Github Enterprise, and would like to reduce the interval.

Please add a setting for this in the DVCS config page.

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As for workarounds, editing the service through the web UI doesn't work (JRA-36774), and editing the service interval in the database and restarting doesn't work either, as something resets the interval to 60 minutes on start.