#5868)

When updating a single uid predicate with a reverse index, the existing entry in the
reverse index should be deleted first.

Fixes DGRAPH-1738

(cherry picked from commit 5b70fe8)

We used to compute reverse count indexes twice while reindexing.
First, while computing reverse edges, and then later while explicitly
computing reverse count index. Now, we use a different function
while reindexing reverse edges and do not compute the reverse
count indexes there any more.

#5868)

When updating a single uid predicate with a reverse index, the existing entry in the
reverse index should be deleted first.

Fixes DGRAPH-1738