Clean up IsWindows check, add hasProc check

Patch by Blake Eggleston; reviewed by jmckenzie for CASSANDRA-8452
Expose thread pool max in metrics

Patch by Chris Lohfink; reviewed by jmckenzie for CASSANDRA-8328
Remove linux-centric references from windows launch scripts

Patch by jmckenzie; reviewed by pthompson for CASSANDRA-8223
fix race on sstable ref counting with SSTableScanner
fix import ordering
simplify getScanner with tokenrange