S3 sync should use --exact-timestamps by default

It's very easy for people to miss that `--exact-timestamps` is necessary for `aws s3 sync` to produce correct results, especially since the default settings are the opposite of what common tools like rsync use. `--exact-timestamps` should be enabled by default so `sync` matches common English usage and the description in the man page which is currently incorrect:

> Recursively copies new and updated files from the source directory to the destination.

If someone has an odd case where this is an optimization the current behaviour could be offered as a `--no-exact-timestamps` or, better, following rsync's lead with `--size-only` to make the implications more obvious.