Feature flags

We need a way to enable/disable certain features using flags.
The flags are properties in the form:
org.infinispan.featurename
any META-INF/infinispan-features.properties resources found on the classpath will be loaded and aggregated, and any property that is set to false will not be available.
The Features object will be initialized by global configurations (GlobalConfigurationBuilder and Hot Rod ConfigurationBuilder) and can be queried in configuration builders' validate methods to decide whether features can be used or not.