Handle negative timestamps in writetime method

patch by slebresne; reviewed by thobbs for CASSANDRA-8139
Update CQLSSTableWriter to allow parallel writing of SSTables on the same table within the same JVM

Patch by Carl Yeksigian, reviewed by Benjamin Lerer for CASSANDRA-7463
Fix totalDiskSpaceUsed calculation

patch by yukim; reviewed by krummas for CASSANDRA-8205