A few enhancements

[SLMT]

• Improve the buffer replacement strategy.
  • Add a reference bit to every buffer so that the clock replacement strategy provides more stable performance.
• Remove the cached string from BlockId
  • Since toString of BlockId is rarely called, pre-computing the string not only does not help but also wastes resources.
• Improve IndexUpdatePlannerTest
  • The name of the indices in the test case might be accidently treated as removable temp files.
• Improve TimerStatistics.
• Remove an unstable test case.
• Update JUnit to 4.13.2