This makes decisions made by the scheduler consistent with decisions
made on table creation, with regard to tablet count.
We want to avoid over-allocation of tablets when table is created,
which would then be reduced by the scheduler's scaling logic. Not just
to avoid wasteful migrations post table creation, but to respect the
per-shard goal. To respect the per-shard goal, the algorithm will no
longer be as simple as looking at hints, and we want to share the
algorithm between the scheduler and initial tablet allocator. So
invoke the scheduler to get the tablet count when table is created.
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