Changing the stack size and memtable space

Change the stack size to improve performance under Tomcat.

Some Solr users have reported that increasing the stack size improves performance under Tomcat. To increase the stack size, uncomment and modify the default -Xss256k setting in the cassandra-env.sh file. Also, decreasing the memtable space to make room for Solr caches might improve performance. Modify the memtable space using the memtable_heap_space_in_mb and memtable_offheap_space_in_mb properties in the cassandra.yaml file.