FilterStatisticsValuesGenerator retains references to managers through static final references

The FilterStatisticsValuesGenerator maintains a map of static final objects that reference managers retrieved from ComponentManager.

However, when the managers are refreshed throughout the JIRA lifecycle - e.g. ManagerFactory.globalrefresh() called after an import - the manager references held in FilterStatisticsValuesGenerator are not updated. Hence, the statistics displayed after an import will be out of date as the original manager references are used.

The FilterStatisticsValuesGenerator should be modified to be singleton and each client should declare its dependency on FilterStatisticsValuesGenerator through its constructor.