Release 6.20.0: Switch to Coyote socket handler /Tomcat's AJP state machine)

[changerequest]

After stability problems are addressed with v6.20rev24, we can now safely switch to Coyote socket handler /Tomcat's AJP state machine).

The property AJP_COYOTE_SOCKET_HANDLER in file 'ajp.properties' is therefore set to TRUE.