Improve Settings#get lookup for camel case support

Today, if we miss on a get on setting, we try its camel case version. The assumption is that in our code, we never use camel case to lookup a setting, but we want to support camel case if the user provided one.
This can be expensive (#toCamelCase) when the get on the setting is done in a tight call, which is evident when running the allocation deciders as part of the reroute logic.
Instead of doing the camel case check on get, prepare an additional map that includes all the settings that are provided as came case, and try and lookup from it if needed.
Should it be an empty map instead of null when empty?