As part of async summarization work, rename snapshotCore() and make it work with summaries

[vladsud]

Currently, SharedObject has this signature - this is what is expected from each DDS:

protected snapshotCore(serializer: IFluidSerializer): ITree;

While public API is the following, with SharedObject doing conversion:

public summarize(fullTree: boolean = false, trackState: boolean = false): ISummaryTreeWithStats;

We need to

• remove term "snapshot" from our vocabulary
• Remove usage of ITree in favor of ISummaryTreeWithStats, by pushing this deeper into each DDS.

This is a fork of #3929, part of #4683

Please assign to write milestone as appropriate