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

Pulp v2 displays a significant slow down when sending incremental progress report updates...i.e. if downloading a 1MB RPM multiple invocations will be executed as pycurl grabs a few KB at a time.  

To help lessen the burden on Pulp v2 grinder is introducing a flag, defaults to False, for sending incremental progress updates.

The investigation leading to this change is document in the below wiki page:
https://fedorahosted.org/pulp/wiki/PerformanceTesting/SyncPerformance_comparison_v1_and_v2