Reduce number of data directory scans during startup

ATM we scan each data directory four times. We could easily reduce that to at least two, maybe to one.

1. pre-flight (startup tests) scrub
2. pre-flight (startup tests) sstable min version
3. ColumnFamilyStore.createColumnFamilyStore
4. ColumnFamilyStore.<init> (if loadSSTables==true)

First two pre-flight tests could be combined to one and 3+4 could also be combined, as both appear at pretty related code paths.