[Improvement] using KeyMayExist instead of Get when visit rocksdb

[wangbo]

Acoording to db.h comment
KeyMayExist is lighter-weight than Get

// If the key definitely does not exist in the database, then this method
  // returns false, else true. If the caller wants to obtain value when the key
  // is found in memory, a bool for 'value_found' must be passed. 'value_found'
  // will be true on return if value has been set properly.
  // This check is potentially lighter-weight than invoking DB::Get(). One way
  // to make this lighter weight is to avoid doing any IOs.
  // Default implementation here returns true and sets 'value_found' to false
  virtual bool KeyMayExist(const ReadOptions& /*options*/,
                           ColumnFamilyHandle* /*column_family*/,
                           const Slice& /*key*/, std::string* /*value*/,
                           bool* value_found = nullptr) {
    if (value_found != nullptr) {
      *value_found = false;
    }
    return true;
  }