(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.
I think the +1 should be applied to the result of `(* (connector-count config) (threads-per-connector num-cpus))` down in `calculate-minimum-threads` rather than here.  Doing it here would apply the +1 for each connector but Jetty's calculation only does it once, to the sum of all selectors and acceptors for all connectors.  See https://github.com/eclipse/jetty.project/blob/jetty-9.1.1.v20140108/jetty-server/src/main/java/org/eclipse/jetty/server/Server.java#L307-L317.