[mysql] prevent cascading deletes on jobs by keep_alive_at

[dlen]

keep_alive_at function uses replace to update the worker row and that forces the jobs child rows to be deleted due to their foreign key relationship. This behavior is due MySQL REPLACE deletes first a row when duplicated.

Using INSERT .. ON DUPLICATE KEY solves the issue. Jobs are not deleted anymore when the keep_alive_at runs. Users should be responsible about truncating the jobs table with this approach. Fixes #48

[geofmureithi]

Thanks for this catch, this is one of the longest running issue on this repo.