Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=50903

Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=50903
If a connector is stopped, not not process any keep-alive connections. The exact behaviours are:
- HTTP BIO - blocks until request line is read and then returns 503
- HTTP NIO - uses polling, so returns 503 immediately connector is stopped
- HTTP APR - blocks until request line is read and then returns 503
- AJP BIO - blocks until request packet is received and then returns 503 - subsequent requests will timeout
- AJP APR - no change - subsequent requests will timeout

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1081882 13f79535-47bb-0310-9956-ffa450edef68