Cache frequently used PropertySets

JAG uses JIRA's property sets to store configurations. We frequently access these property sets in various services (ex. SprintCustomFieldServiceImpl).

However we don't cache these property sets, thus resulting in a lot of unnecessary db access to retrieve the data for the sets each time we want to read a property.

We should apply an appropriate caching strategy for these property sets.

This has appeared to cause significant bottlenecking in a customer instance, where almost every JIRA Agile thread is blocked on a thread waiting on a response from the database due. Screenshot of threads attached.