Perf Fix CreateEtags

[ElizabethOkerio]

This PR fixes some of the issues raised in this PR: #1111:
I've made the following updates to the CreateEtag method:

• Set the initial value of the properties dictionary to null and initialize properties only within the foreach loop. This approach guarantees that the dictionary is instantiated only when concurrency properties are present.
• I excluded the OrderBy operation from the IEnumerable of concurrency properties to eliminate the cost associated with OrderedEnumerable.MoveNext(). Consequently, the existing approach retrieves the concurrent properties, orders them, and subsequently inserts them into a dictionary. If the dictionary preserves the order, the crucial factor lies in the manner in which properties are added to the dictionary rather than whether they are sorted before being added to the dictionary. @xuzhg what are your thoughts on this?
• Added a null check for the properties, ensuring that the CreateEtag method is invoked only when the properties are non-null.
  These changes have the following CPU Usage improvements:
  Before changes:

After changes:

[gathogojr]

I feel a certain type of way about checking whether the properties dictionary is null in each iteration? More CPU cycles and feels like we're sacrificing one thing for the other. If we must go this route, then the following would also be candidate:

IEnumerator<IEdmStructuralProperty> concurrencyPropertiesEnumerator = concurrencyProperties.GetEnumerator());

if (concurrencyPropertiesEnumerator.MoveNext())
{
    properties = new Dictionary<string, object>();

    do
    {
        IEdmStructuralProperty etagProperty = concurrencyPropertiesEnumerator.Current;
        properties.Add(etagProperty.Name, resourceContext.GetPropertyValue(etagProperty.Name));
    }
    while (concurrencyPropertiesEnumerator.MoveNext());
}

At least this way we're not checking if properties dictionary is null in each iteration.
Whether the code is clean and easy-to-follow is a different matter altogether...

[ElizabethOkerio]

In most cases we do not have concurrency properties..

[habbes]

For what it's worth, I think gathogo's suggestion of using MoveNext() instead of doing a null-check on each iteration wasn't that bad. But since this code path isn't that common, not sure it makes material difference.