(TK-13) Dynamic default for max-threads set

On machines with a large number of cores, the default thread pool size
of 100 is too small. This commit will calculate the minimum number of
threads needed to run, and use that as the max-threads value if one
isn't already set in the configuration.
Yeah, that's a good idea.  Actually, the work I had started for TK-82 would, in part, involve setting the `minThreads` value.  I have to do that because I need to use a different `QueuedThreadPool` constructor in order to pass in the backing queue but which also mandates that a `minThread` value be specified.  I didn't see a need yet to expose this through the TK webserver configuration, so I just went with the default value that Jetty 9.1.1 uses - 8.  See https://github.com/eclipse/jetty.project/blob/jetty-9.1.1.v20140108/jetty-util/src/main/java/org/eclipse/jetty/util/thread/QueuedThreadPool.java#L73.  Ok with you if we defer this question out to the work for TK-82?