ARTEMIS-6068 Broker plugins do not receive updated properties on broker.xml reload

[mbengtsson]

Jira: https://issues.apache.org/jira/browse/ARTEMIS-6068

This closes a gap in the existing broker.xml reload model. Artemis already reloads several broker-managed configuration areas, but broker plugin properties are only delivered during plugin initialization.

Without a dedicated reload callback, plugins that need to react to changed configuration must implement their own file watching and parsing logic outside the broker.

This change adds propertiesReloaded(Map<String, String>) as a callback on ActiveMQServerBasePlugin and invokes it for matching already-running plugin instances during broker.xml reload.

The existing broker-plugin XML format is preserved. Matching is by class name because broker-plugin entries currently have no name/id attribute.

Tests: BrokerPluginReloadTest

[jbertram]

I'm concerned that there's a fundamental disconnect between the current methods used to manage broker plugins and the new methods you've added. Consider the following for a user managing an embedded broker:

1. They might call Configuration#setBrokerPluginConfigurations and then expect those plugins to be deployed when they start their broker when, in fact, they won't.
2. They might call Configuration#registerBrokerPlugin and then expect to get this plugin's configuration when they invoke Configuration#getBrokerPluginConfigurations later, but they won't.

Additionally, these new methods store & manage broker plugin configuration data in different data structures from the existing methods so it's likely that things will get out of sync.

Ultimately, I think this functionality must be implemented within the confines of the existing API. If this is not possible then we'll have to break the API in order to support this functionality which can only happen in a major release (e.g., 3.0).