Queries that look for stories with tags take significant amount of
time without this change. One trivial query for active stories with
one tag dropped from 7m19s to 2.7s.
Story: #2001939
Task: #15055
Change-Id: Id1783a3cf355f11b2037f4e0ada9514993820bca
Signed-off-by: Doug Hellmann <doug@doughellmann.com>