Look into better default file_cache_size for 2.2

CASSANDRA-8464 added support for mmapped decompression where in version <= 2.1 the reads were all decompressed in standard heap buffers.

Since the usage of the file_cache is based solely on the buffer capacity we should/can make this much larger in 2.2 when the disk access mode is mmap.

The downside of this cache being too small is made worse by 8464 since the buffers are mmapped/unmapped causing explicit page faults.