Rearrange default services to better anticipate another store.
Make cost estimating paths more consistent.
Split out CostModel part that obviously depends on storage. Add a factory controlled by a service to get right one.
Add a hook for tweaking cost estimates during testing.
Enable the new service.
Sketch how a random cost model might intervene.
merge MMcM: Add a cost model factory

Merge pull request #1053 from MMcM/sql-layer/cost-model-factory
https://github.com/FoundationDB/sql-layer/pull/1053