Use a unique constraint to prevent duplicate cron jobs

Adds a new params column to the cda_cacheControl table which contains
the parameters as a string.

Set a unique constraint on (cdaFile, dataAccessId, userName, cronString,
params) to prevent duplicate cron jobs from being created.
[CDA-70]: Prevent duplicate cron jons from being created in schedule
Merge pull request #35 from pamval/master

CDA-70