Sending server meta data should be separated from the agent's lifecycle

[Xylus]

The agent's lifecycle (and sending agent information) is currently controlled by plugins. Each plugin must specify if it will control the agent's lifecycle, and call Agent.start() manually if it does.

For example, a Tomcat 6 application is modified to invoke Agent.start() after org.apache.catalina.core.StandardService.start() method completes, and only then would the agent sends its information along with the server's meta data.

The agent's information should be sent regardless of the enclosing application's status, and this requires adding a way for the application to notify that it has finished gathering the server's meta data.

[Xylus]

Making further modifications, It would be better for PinpointBootstrap to govern the agent's lifecycle (ie. start/stop methods).