switch from newing up a ICollection ot a IReadOnlyCollection

[ttstanley]

Context

There were lots of allocations due to newing up a ReadOnlyCollection just to fufill a ICollection contract even if it was internal.

Changes Made

• For internal usage I moved over to returning IReadOnlyCollection instead of creating a new ReadOnlyCollection that can be used with ICollection.
• For public api's that couldn't change I waited till the actual return time to create a ReadOnlyCollection
  

Testing

Took traces of build before my changes and after my changes.
Before (ReadOnlyCollection 33 Megs of allocations)

After (ReadOnlyCollection 5 Megs of allocations)

Notes