improve s3a committer stats collected

we can improve stats collected in the s3a committer and saved to the JSON.

key ones

1. of task manifests read; duration of loads
2. size of each manifest

I think we would also benefit if we could set the commit thread pools to be big -but then shared across all jobs (i.e. demand-created thread pool in s3a fs). that would allow for a pool size of say, 500, but still support many jobs actively committing at same time (busy spark driver)
finally: should file commit pool size be > size of pool of manifest readers. I think it could be, but the ratio should be fairly low.