U1db implementation

We have been preferring to define keys somewhere else and use the variables to avoid typ0s. Something like:

    # defined somewhere else and importable everywhere
    TARGET_REPLICA_UID_KEY = "target_replica_uid"
    TARGET_REPLICA_GEN_KEY = "target_replica_generation"
    TARGET_REPLICA_TRANS_ID_KEY = "target_replica_trans_id"

    # on current document
    from somewhereelse import TARGET_REPLICA_UID_KEY
    from somewhereelse import TARGET_REPLICA_GEN_KEY
    from somewhereelse import TARGET_REPLICA_TRANS_ID_KEY
    
    def get_target_info():
        info[TARGET_REPLICA_UID_KEY] = str(uuid.uuid4())
        info[TARGET_REPLICA_GEN_KEY] = 12
        info[TARGET_REPLICA_TRANS_ID_KEY] = str(uuid.uuid4())