Expose additional web subsystem metrics

A user has requested that some further web subsystem metrics be exposed via the management interface:

"When we run JVM with "-Dorg.apache.tomcat.util.ENABLE_MODELER=true"
option, we can get currentThreadsBusy via JMX.
e.g.
ObjectName poolObjName = new
ObjectName("jboss.web:type=ThreadPool,name=ajp-0.0.0.0-0.0.0.0-8009");
ajpCurrentThreadsBusy = ((Integer) pfServer.getAttribute(poolObjName,
"currentThreadsBusy")).intValue();

It is nice if we could get the information via CLI like:
/subsystem=web/connector=ajp13:read-attribute(name=current-threads-busy)

Additionally, We also would like to get activeSessionCount. We can get
the information via JMX like:
ObjectName sessionObjName = new
ObjectName("jboss.web:type=Manager,path=/tpcw,host=default-host");
activeSessions = ((Long) pfServer.getAttribute(sessionObjName,
"activeSessionCount")).intValue();

And it is nice if we could get the information with CLI like:
/deployment=tpcw.war/subsystem=web/servlet=TPCWServletWithEncoding:read-attribute(name=active-session-count)"

I don't think the active-session-count attribute belongs under the "servlet" resource though, since sessions are not associated with servlets. A new resource for the session manager would be needed.