Split LockableScore responsibilities between read & write operations

Split LockableScore responsibilities between read & write operations

    - Split Score from LockableScore to ScoreLookUp to handle read
      operations and ScoreUpdate to handle write operations
    - Change all struct that implemented Score to implement ScoreLookUp
      and/or ScoreUpdate
    - Change Mutex's to RwLocks to allow multiple data readers
    - Change LockableScore to Deref in ScorerAccountingForInFlightHtlcs
      as we only need to read
    - Add ScoreLookUp and ScoreUpdate docs
    - Remove reference(&'a) and Sized from Score in ScorerAccountingForInFlightHtlcs
      as Score implements Deref
    - Split MultiThreadedScoreLock into MultiThreadedScoreLockWrite and MultiThreadedScoreLockRead.
      After splitting LockableScore, we split MultiThreadedScoreLock following
      the same way, splitting a single score into two srtucts, one for read and
      other for write.
      MultiThreadedScoreLock is used in c_bindings.