Make disableDefaultExceptionMapperRegistration public in StandardExceptionMappers

[sleberknight]

The disableDefaultExceptionMapperRegistration method has been around for many years, but is private. To provide more flexibility to applications which might want to disable Dropwizard's default exception mappers programmatically instead of static, repetitive YAML configuration, make this method public.

This will allow applications to easily disable Dropwizard's default exception mappers, and then register only the exception mappers they want instead of getting the opinionated set of them that the StandardExceptionMappers#register methods provide.

[sleberknight]

We should also "optimize" this method so that it directly calls the setter method if the ServerFactory is an instance of AbstractServerFactory. Since Dropwizard has provided only two implementations (DefaultServerFactory and SimpleServerFactory) for many, many years, they probably won't be changing this anytime soon. And, doing this avoids the overhead of the reflective call. Last, while it is highly unlikely we'll ever need it, we can keep the existing reflective code but only invoke if the ServerFactory is not an instance of AbstractServerFactory.