Fix ColumnFamily opening race

(Moving from CASSANDRA-5151)

Currently, MeteredFlusher is scheduled inside static block of ColumnFamilyStore, and it accesses all ColumnFamilyStore when it runs every 1 sec. Scheduling is done when JVM first load ColumnFamilyStore class, so after that, there is always a chance to open SSTables before doing scrub directory/remove compaction left overs.
We should move the content of static block at the end of CassandraDaemon setup.