Big Switch: Remove consistency hash on full sync

Clears the consistency hash before performing a full
backend synchronization. Since all of the data is being
replaced, there is no reason to include this and it
currently requires special-casing to handle.

Closes-Bug: #1328233
Change-Id: Iace766d869dc78b041d3a5464e728b872c8347c2
(cherry picked from commit e60670e5a202870693f7485a1da4411f4faaf36d)
Merge "Big Switch: Remove consistency hash on full sync" into stable/icehouse