It's slightly annoying to plumb Optional[MentionBackend]
down the stack, but it's a one-time change.
I tried to make the cache code relatively unobtrusive
for the single-message use case.
We should be able to eliminate redundant stream queries
using similar techniques.
I considered caching at the level of rendering the message
itself, but this involves nearly as much plumbing, and
you have to account for the fact that several users on
your realm may have distinct default languages (French,
Spanish, Russian, etc.), so you would not eliminate as
many query hops. Also, if multiple streams were involved,
users would get slightly different messages based on
their prior subscriptions.
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