add file_cache_size_in_mb setting
patch by pyaskevich and jbellis for CASSANDRA-5661
merge from 1.2
Revert "add file_cache_size_in_mb setting"

This reverts commit dfe49376097cf73c04c0d8506782263a2e820cb0.
Merge branch 'cassandra-1.2' into cassandra-2.0
merge from 1.2