Improve perf for iterating over tracked entities

Part of #8898
Fixes #14231

Investigation into DbSet.Local shows that:
* Iteration over tracked entities had a lot of LINQ code; fixed by un-LINQing
* Multiple composed iterators cause slow-down; fixed by giving LocalView it's only IStateManager method
* Filtering causes slow-down; fixed by splitting storage into multiple dictionaries. Makes lookup for entity instance slightly slower.
* Calculate Count lazily

This means that DbSet.Local iteration is now about 3x faster, although it depends a lot on what is being tracked. Getting an ObservableCollection and then iterating over that _once_ is slower than just iterating over the entries once. Iterating over the ObservableCollection multiple times is still faster than iterating over DbSet.Local multiple times. This seems like a reasonable trade-off. If the application does heavy iteration, then creating a new collection for that seems reasonable.

With regard to #8898, the layering we have still seems correct, so moving forward with the breaking changes there.

src/EFCore.Relational/Migrations/Internal/MigrationsModelDiffer.cs

@@ -1725,12 +1725,9 @@ protected virtual IEnumerable<MigrationOperation> GetDataOperations()
         {
             if (_sourceStateManager != null)
             {
-                foreach (var sourceEntry in _sourceStateManager.Entries.ToList())
+                foreach (var sourceEntry in _sourceStateManager.ToListForState(added: true))
                 {
-                    if (sourceEntry.EntityState == EntityState.Added)
-                    {
-                        sourceEntry.SetEntityState(EntityState.Detached);
-                    }
+                    sourceEntry.SetEntityState(EntityState.Detached);
                 }
             }
 

src/EFCore/ChangeTracking/Internal/ChangeDetector.cs

@@ -127,12 +127,11 @@ public virtual void DetectChanges(IStateManager stateManager)
         {
             _logger.DetectChangesStarting(stateManager.Context);
 
-            foreach (var entry in stateManager.Entries.Where(
-                e => e.EntityState != EntityState.Detached
-                     && e.EntityType.GetChangeTrackingStrategy() == ChangeTrackingStrategy.Snapshot).ToList())
+            foreach (var entry in stateManager.ToList()) // Might be too big, but usually _all_ entities are using Snapshot tracking
+
             {
-                // State might change while detecting changes on other entries
-                if (entry.EntityState != EntityState.Detached)
+                if (entry.EntityType.GetChangeTrackingStrategy() == ChangeTrackingStrategy.Snapshot
+                    && entry.EntityState != EntityState.Detached)
                 {
                     LocalDetectChanges(entry);
                 }