Prevent hash_tree_root caching of mutable objects

[ericsson49]

CachingBeaconChainSpec::hash_tree_root can be passed with a mutable object, e.g. MutableBeaconState or WriteList/WriteVector. Thus it can return wrong value, if the mutable object is modified, since the caching code uses Object identity (BeaconStateImple::hashCode is not overridden). There are several possible ways to solve the problem:

1. implement proper BeaconStateImple::hashCode to match the BeaconStateImple::equals implementation
2. Invalidate cache based on updates
3. Do not cache mutable objects. This will require implementing an appropriate flag/interface to manifest (im)mutability.