Add RepeatedField<T> support for RocksDbStore

[Havret]

It should be possible to use RepeatedField as top level value for RocksDbStore

rocksDbBuilder.AddStore<string, RepeatedField<User>, UsersStore>("users-store");

public class UsersStore : RocksDbStore<string, RepeatedField<User>>
{
    public UsersStore(IRocksDbAccessor<string, RepeatedField<User>> rocksDbAccessor) : base(rocksDbAccessor)
    {
    }
}