We should emit number of sstables in each level from JMX

We should add methods to this Mbean org.apache.cassandra.db.ColumnFamilyStoreMBean

These metrics will be helpful to see how sstables are distributed in different levels and how they move to higher level with time.
Currently we can see this by looking at the json file but with JMX, we can monitor the historic values over time using any monitoring tool.