DAL: revisit protocol parameters encoding

       (req "feature_enable" Data_encoding.bool)
       (req "number_of_slots" Data_encoding.int16)
       (req "number_of_shards" Data_encoding.int16)
       (req "endorsement_lag" Data_encoding.int16)
       (req "availability_threshold" Data_encoding.int16)
       (req "slot_size" Data_encoding.int31)
       (req "redundancy_factor" Data_encoding.uint8)
       (req "page_size" Data_encoding.uint16))

• Integers should be uintX
• endorsement_lag should be uint8 (and probably availability_threshold as well)