In Review Board, a pending review object or pending reply object is

intended to be unique to each user (that is, a user may only have one

pending reply to a given review at a time). Unfortunately, we do hit

cases where network lag, overloaded webservers, or slow databases can

create situations where multiple API requests come in and end up

creating multiple objects. This then results in errors about

MultipleObjectsReturned when attempting to fetch later.
We already have logic for reviews to detect this situation and coalesce

duplicates into a single review. This change implements something very

similar for pending review replies, leveraging the same coalescing

logic.

Ran unit tests. Before the fix, this would hit the

MultipleObjectsReturned error. After, the new test passes successfully.