[UnifiedDataStore] Use pre-increment of an iterator to improve performance.

In general, a post-increment operation of an iterator creates a temporary
object. Since it is often a class, its constructor is executed. As a result,
a post-increment operation is slightly slower than the pre-increment.
Merge pull request #82 from kz0817/async-face-rest

[UnifiedDataStore] Use pre-increment of an iterator to improve performance.