Enable max-queued-requests by default

Puppet server 5.1.0 added a max-queued-requests setting that will terminate agent requests early when the number of requests waiting for a JRuby instance exceeded a limit imposed by the setting (SERVER-1767). The requests are failed with a HTTP 503 error that includes a Retry-After header set to a randomized number of seconds that the agent should sleep before retrying the request. This randomized sleep allows the server to "re-splay" thundering herds by spacing the agents out without requiring the intervention of human operators.

Currently, max-queued-requests is off by default. But, we should consider changing the default to enabled as the setting not only re-splays agent runs but ensures that some Jetty workers are free to handle requests that do not require a Ruby interpreter — such as performance metric snapsots.

The "ideal" default for this limit is likely some multiple of max-active-jrubies with a hard cap that ensures 25% – 33% of the Jetty worker pool is available to serve non-Ruby requests. We should also consider lowering the default for max-retry-delay from 30 minutes to 5 or 10 minutes so that agent runs are not overly delayed.