test/rgw: Add incremental test of bucket sync run

test/rgw: Add incremental test of bucket sync run

This tests for iterating properly over the generations.

1. Create a bucket and write some objects to it. Wait for sync to
   complete. This ensures we are in Incremental.
2. Turn off the secondary `radosgw`.
3. Manually reshard. Then continue writing objects and resharding.
4. Choose objects so that each generation has objects in many but not
   all shards.
5. After building up several generations, run `bucket sync run` on the
   secondary.
6. Verify that all objects on the primary are on the secondary.

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>