Explore passing workers into P2P merges

see #557

We should be able to do this more elegantly as soon as we materialise the graph on the scheduler to avoid dropping the results.

Start with issue #557 and the scheduler code for P2P merges. Trace how workers and merge results are handled before the graph is materialised, especially where results may be dropped. Done means establishing an agreed design that passes workers into P2P merges without losing results.

Written by the indexing model from the issue text.