Minor improvements in Cassandra shutdown and startup logs

To improve a DBA experience the following log messages would be nice to add/adjust:

• on shutdown: an explicit message at the end of Cassandra shutdown
• on startup:
  • print the time spent to load prepared statements
  • print the time spent to load repair session information and the number of loaded session records
  • print the time spent to apply commit log

It would help with assessment of possible delays in startup/shutdown of Cassandra. For example, recently I observed a delay in Cassandra startup and from logs it was not clear was it caused by loading of prepared statements or repair service init.