Stream results to disk as they come in.

Results are written to disk without re-serializing them, and in batches
of ~1 MB at a time.  This disk I/O is done in a dedicated OS thread in
ssbench-master, so the I/O blocking won't interfere with processing
results and handing out work.  fixes #62.

Also include more scenario parameters (client concurrency and operation
count) in auto-generated results filename.  fixes #64.