handle superseded messages properly for incremental mode in GetThreadNonblock CORE-8523

[mmaxim]

There is a problem in the INCREMENTAL mode of GetThreadNonblock where it does not include messages in the remote result that were modified (by a superseder) from the cached result. This affects edits and deletes of messages that we have cached. Patch does the following to fix the situation:

1.) Change mergeLocalRemoteThread to include messages from the remote result that have a SupersededBy value that is different than what we sent up for the cached result.
2.) Fix how resolveHoles works in HybridConversationSource. Previously, it would try to modify the thread that we fetched from storage in-place. However, it didn't do anything for superseded messages, and so the results we got back from a "holey" cache hit were wrong. Now instead of modifying the thread in resolveHoles, we just re-fetch from Storage if we get a holey hit and resolve it.
3.) Add a new mode for basicSupersedesTransform to include a "hidden" PLACEHOLDER message instead of just dropping the message for deleted messages. This allows us to communicate to clients of GetThreadNonblock that cached messages have actually been deleted.
4.) Bonus Fix: PullLocalOnly had previously been using a SimpleResultCollector regardless of the query passed in. This made it such that it returned a subset of what Pull would when given the same query. Change this to use a ResultCollector derived from the query.

cc @buoyad

[joshblum]

on any of these calls when you pass unnamed value in to call this can you add /* useDeletePlaceholders */ so it's easier to read in the future?