Previously, there was no logic to check, if a JSR223 script
was modified should be refreshed from the external resource.
* Fix `RefreshableResourceScriptSource` to take care of refreshing
on call `isModified()`
* Add `scriptSource.isModified()` before getting script text
* Introduce `ScriptSourceFactoryBean` to avoid I/O operations, when
there is need to refresh script resource - `refreshDelay < 0`
JIRA: https://jira.springsource.org/browse/INT-3164
INT-3164: change the refresh mutation logic
Remove factory bean - no longer needed.
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