Default to _not_ sending incremental progress reports as an item downloads
This means if you are downloading a 100MB file you will only see a progress update when the item completes
You will not see progress as the actual bits are downloading.

Helps to ease an issue seen with Pulp 2.0.8
Merge pull request #1 from jwmatthews/master

Improves Pulp v2 sync performance from ~180mins to ~60 mins