Update ConfigurationManager with thread-safe methods

Merge ConcurrentConfigurationManager interface into ConfigurationManager
interface. This will force all clients to start using new thread-safe
ConfigurationManager methods.

DefaultConcurrentConfigurationManager is now the new "default"
ConfigurationManager. It simply adds concurrency, while delegating to
ResourceMergingConfigurationManager, which still delegates to the old
default ConfigurationManager (DefaultConfigurationManager)
Use thread-safe ConfigurationManager methods

Ensure that all users of ConfigurationManager are using "default", and
update usage to use new thread-safe methods
Update tests

Update tests to use DefaultConfigurationManager directly. Arguably,
these should be changed to use DefaultConcurrentConfigurationManager,
but they were never updated to use ResourceMergingConfigurationManager
when that was introduced, and the intent of the tests is to test the
functionality in DefaultConfigurationManager, not
DefaultConcurrentConfigurationManager
Update security version to 3.1-SNAPSHOT

Bumped minor version due to changes to ConfigurationManager interface
Delete obsolete files