MediaMetrics: Add copy constructor to TransactionLog

 *

 * The TransactionLog is NOT thread safe.

 */

class TransactionLog {

class TransactionLog final { // made final as we have copy constructor instead of dup() override.

public:

    // In long term run, the garbage collector aims to keep the

    // Transaction Log between the Low Water Mark and the High Water Mark.

                  __func__, highWaterMark, lowWaterMark);

    }

    // The TransactionLog copy constructor/assignment is effectively an

    // instantaneous, isochronous snapshot of the other TransactionLog.

    //

    // The contents of the Transaction Log are shared pointers to immutable instances -

    // std::shared_ptr<const mediametrics::Item>, so we use a shallow copy,

    // which is more efficient in space and execution time than a deep copy,

    // and gives the same results.

    TransactionLog(const TransactionLog &other) {

        *this = other;

    }

    TransactionLog& operator=(const TransactionLog &other) {

        std::lock_guard lock(mLock);

        mLog.clear();

        mItemMap.clear();

        std::lock_guard lock2(other.mLock);

        mLog = other.mLog;

        mItemMap = other.mItemMap;

        return *this;

    }

    /**

     * Put an item in the TransactionLog.

     */