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.
[CDE-198]: handles NullPointerException when trying to resolve role for Anonymous User
Use streamed export on unwrapQuery
re-enable streaming exporter
Avoid data conversion for the NULL values case
- Use step meta class instead of hand-generated XML string
- Fixed streaming export functionality in Pentaho 5
[CDA-63]: Fixing the StreamExporter
Merge pull request #34 from pamval/master

[CDA-63]: StreamExporter
Merge branch 'master' of github.com:webdetails/cda
[CDE-198]: changed from debug log to warning log
Merge pull request #32 from davidmsantos90/master

[CDE-198]: handles NullPointerException when trying to resolve role for ...
[CDA-70]: Prevent duplicate cron jons from being created in schedule
Merge pull request #35 from pamval/master

CDA-70
[CDA-61]: DataAccess type set in cache key
Merge pull request #36 from davidmsantos90/master

[CDA-61]: DataAccess type set in cache key
[CDA-57]: dataAccess list in the Previewer is sorted alphabetically
Merge pull request #37 from davidmsantos90/master

[CDA-57]: dataAccess list in the Previewer is sorted alphabetically
[CDA-61]: Fixed DataAccess type set in cache key
Merge pull request #38 from davidmsantos90/master

[CDA-61]: Fixed DataAccess type set in cache key
Fix exporting to CSV for null values in data