When writing transition updates to a tablet map we must do so on a base
table. A table that is co-located with a base table doesn't have it's
own tablet map in the tablets table, but it only points to the base
table map. By writing to the base table, the tablet migration will be
applied for the entire co-location group.
We add a small helper in storage_service that creates a tablet mutation
builder for the base table, and use it whenever we need to write tablet
mutations.
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