Ckozak/refresh pools memory

This is going to be a bit slower (except for huge orgs).

The goal is to keep less stuff (subscriptions, entitlements, pools) in memory for the duration of a refresh pools run.
breaking that down to holding ids rather than entire objects made it much easier to break the refresh into multiple commits.

Because we sync one subscription at a time, if we break half way though, we will end up better off than we would have by rolling the whole thing back.
Meaning the method no-ops if uow is null?