Better memory support

[olegkovalenko]

MOTIVATION Avoid OOM errors on servers when node --mem option is less than 1/2 total RAM on server.

PROPOSED CHANGE: Extract Xmx ans Xmn from cassandra-jmv-options

• whenever Xmx is not set, calculate it using formula xmx = max(min(1/2 * mem, 1024MB), min(1/4 * mem, 8GB))
• whenever Xmn is not set, calcualte it using formula xmn = min(100 MB * cpu, 1/4 * xmx) update MAX_HEAP_SIZE, HEAP_NEWSIZE in cassandra-env.sh

RESULT: flexible memory configuration

[dmitrypekar]

Merged. Thank you for the update.

[olegkovalenko]

Thanks!