Add support for caching file handles on s3

The file handle cache is currently disabled for S3, as the S3 connector needed to implement proper unbuffer support. Now that https://issues.apache.org/jira/browse/HADOOP-14747 is fixed, Impala should provide an option to cache S3 file handles.

This is particularly important for data caching, as accessing the data cache happens after obtaining a file handle. If getting a file handle is slow, the caching will be less effective.