Change many "maps from stringified ID" to _IdMap

This greatly reduces the number of places in the code where "stringified
IDs" are passed around.

Specifically, changed these maps into _IdMaps:

 - minimongo
   - the main LocalCollection docs map
   - LocalCollection._savedOriginals
   - unordered query results, in several places:
     - query.results
     - query.results_snapshot
     - return value from _getRawObjects
     - as passed to _diffQueryChanges
 - livedata
   - Connection._serverDocuments[collection]
 - mongo-livedata
   - SynchronousCursor._visitedIds
   - return value from SynchronousCursor.getRawObjects
   - PollingObserveDriver._results (when unordered)