--parallel option to use gnu parallel for the merge step

This PR introduces a "--parallel" option to carbon-sync.    "--parallel" will shell out to gnu parallel in order to parallelize the whisper-fill invocations.

For our workloads, the speedup is about 5x during the merge phase, and about 2x overall.   
Obviously YMMV depending on number of cores available, etc.

TODO - would be nice to pass arguments to gnu parallel.