"Dynamic" SET_PARAM options in ep-engine are not all modifiable via epctl

In ep-engine the SET_PARAM implementation (EventuallyPersistentEngine::setParam) is a hand-written code to check for each dynamically configurable config parameter.

As such, it is easy to omit a code for particular dynamic config param and not be able to change it via epctl.

For example, flusher_batch_split_trigger is a dynamic config param and the flusher itself can handle it changing dynamically, however it has been omitted from the SET_PARAM(flush) handler - EventuallyPersistentEngine::setFlushParam and hence cannot be modified on the fly. I expect an audit of the config parameters will identify more...

We should:

1. Add tests which verify that every dynamic config parameter in configuration.json can indeed to changed via SET_PARAM, and/or
2. Modify EventuallyPersistentEngine::setParam and related functions to be data-driven / auto-generated from configuration.json, enforcing that all params marked as dynamic are indeed dynamically-modifiable.