Fix for ChangeSetMergeTracker so that it correctly works with Value Types

[dwcullop]

Description

• Addresses [Bug]: ReferenceEquals breaks value types #931 by fixing ChangeSetMergeTracker (the basis for multiple operators including MergeManyChangeSets, MergeChangeSets, and TransformManyAsync)
• Replaces most instances of ReferenceEquals with EqualityComparer<T>.Default.Equals
• A specific helper method was created to use when ReferenceEquals is explicitly needed (which will gracefully handle value types)
• Added unit tests to verify the fix (tests failed without the additional changes)

Other Fixes

• Updates ToObservableOptional with a more efficient implementation that does not use ReferenceEquals

[Al12rs]

Bump, for rebase and review, feedback above seems to have been addressed

[dwcullop]

Sorry I was on vacation the past 10 days. It will now just use the given comparer or EqualityComparer.Default (if not provided).