httpd should remove workers who crashed

When worker W crashes, httpd will not remove it from its worker tables and still try to send requests to W.

We should implement something like ping_mode=I (interval), ping_timeout and connection_ping_interval in mod_jk [1].

It should work as follows:

• W crashes
• httpd waits for connection_ping_interval of idle time, then starts checking the connection to W every ping_timeout seconds (or ms)
• When the connection to W is broken, httpd should remove W from its worker table and stop the connection checker
• When (if !) W is started again, mod-cluster will register it again with httpd

The configuration above should be done entirely on the AS side, e.g. in HAModClusterConfig. maybe we can find just 1 attribute rather than 3.

[1] http://tomcat.apache.org/connectors-doc/reference/workers.html