The main optimization points:
1. Use std::unordered_set instead of std::set, and use RowsetId.hi as RowsetId's hash value.
2. Minimize the scope of SpinLock in UniqueRowsetIdGenerator.
Profile comparation:
* Run UniqueRowsetIdGeneratorTest.GenerateIdBenchmark 10 times
old version | new version
6s962ms | 3s647ms
6s139ms | 3s393ms
6s234ms | 3s686ms
6s060ms | 3s447ms
5s966ms | 4s127ms
5s786ms | 3s994ms
5s778ms | 4s072ms
6s193ms | 4s082ms
6s159ms | 3s560ms
5s591ms | 3s654ms