Use updatable settings for cache

[ebernhardson]

Currently attempting to change the size of the model cache from the cluster settings api results in an error. I don't see any reason this needs to be static. The easy part of this is adding a call to ClusterService::addSettingsUpdateConsumer on init to receive the updates, slightly harder will be adjusting the cache to handle resizing.

PUT /_cluster/settings
{"transient": {"ltr.caches.max_mem": "50mb"}}

{"error":{"root_cause":[{"type":"illegal_argument_exception","reason":"transient setting [ltr.caches.max_mem], not dynamically updateable"}],"type":"illegal_argument_exception","reason":"transient setting [ltr.caches.max_mem], not dynamically updateable"},"status":400}

[jettro]

I'd like to pick this one up. Already made some changes to make the consumer and verified I get a callback when changing the property. SO next step is to decide what to do. Will come up with some ideas, but of course open to suggestions from everybody what they feel should happen when changing one of the properties.