Add in a slightly different behavior for dispatch.

Add in a slightly different behavior for dispatch.
dispatch() will return immediately, according to spec. But if dispatch is called from the tomcat worker thread, then the dispatch will not happen until the worker thread has backed out of the call. This prevents multiple threads being launched into the same request/response and causing potential concurrency issues.
The Async state machine guarantees that multiple dispatch can not be called.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@927182 13f79535-47bb-0310-9956-ffa450edef68