common_enums: removed remaining Serializer references; doc tidy.
Datastore: removed move support, require impl to hide (de)initialization.
config: added MSG_SCOPED defs, unscoped THROW_ERROR & String variant.

config:
    added HORD_MSG_SCOPED_CLASS, HORD_MSG_SCOPED_FUNC, HORD_MSG_SCOPED_FQN;
    added HORD_THROW_ERROR, HORD_THROW_ERROR_S;
    simplified HORD_THROW_ERROR_SCOPED defs.
test: moved dummy classes and utilities to test/common/.