Summary:
Remove the ticker count because:
* Having to reset the ticker count in WriteImpl is ineffiecent;
* It doesn't make sense to have it as a ticker count if multiple db
  instance share a statistics object.
Closes https://github.com/facebook/rocksdb/pull/1531

Differential Revision: D4194442

Pulled By: yiwu-arbug

fbshipit-source-id: e2110a9