Lazy-create DeltaMemStores on first update

Currently DeltaTracker::DoOpen creates a DeltaMemStore for each DRS. If we assume that most DRS don't have any deltas, this ends up wasting quite a bit of memory. Looking at one TS in a production cluster, about 1GB of the ~14G heap is being used by DMS. Of that, 464MB is data and the remainder is overhead.

This would likely improve other code paths too to fast-path out any DMS-related code.