Reduce number of state parameters passed around in data distributor

Based on discussion of https://github.com/apple/foundationdb/pull/3821, many data distributor functions have a lot of parameters, which harms readability. These parameters can be combined into state classes, and references to this state class can be passed around, reducing the length of function signatures.

Start with the discussion in pull request 3821 and trace the data distributor functions it discusses. Identify which parameter groups can become shared state classes; the work is done when those functions pass the state references instead of long parameter lists while preserving existing behavior.

Written by the indexing model from the issue text.