Cache IdentifiedUser across PrologEnvironments

When running multiple rules for the same change, the set of
reviewers is identical as ChangeControl goes up the project
tree and runs rules in parent projects. Recycle all of the
IdentifiedUser objects that were made in one invocation for
the next one, to avoid looking them up again in AccountCache.

Change-Id: I4031a3f8f9eb0328f0abbbfce1f38eb67b4877d3
2 files changed