Allow using `Features` as globals

[clarisma]

Because of the indeterminate static initialization order in C++, it is currently not safe to use Features as global variables.

Features relies on a a global hashmap in order to ensure that only one FeatureStore per process is opened for any given GOL. When a global Features object is destroyed, its associated FeatureStore is removed from this hashmap. However, since destruction order of globals is indeterminate, this hashmap may already have been destroyed, resulting in a crash.

To avoid this, we would need to wrap the hashmap in a class that maintains an "alive" flag (set to true by constructor, cleared by destructor). Once that flag is cleared, the hashmap must no longer be accessed.