Fixed ambiguous overloads for `ObservableCacheEx.ExpireAfter()` and `ObservableListEx.ExpireAfter()`.

[JakenVeina]

Also moved EnsureUniqueKeys() into proper order.

Currently, the following code produces an ambigous method error at compile-time, for the call to .ExpireAfter().

var source = new SourceCache<string, int>(x => x.GetHashCode());

var result = source.ExpireAfter(_ => TimeSpan.FromSeconds(1));

Since any fix for this would be a breaking change, I've implemented it by just consolidating all of the overloads into one method, for simplicity.

The issue itself is relatively minor, as it can be easily worked-around by doing...

var source = new SourceCache<string, int>(x => x.GetHashCode());

var result = source.ExpireAfter(
    timeSelector: _ => TimeSpan.FromSeconds(1),
    interval: null);

or

var source = new SourceCache<string, int>(x => x.GetHashCode());

var result = source.ExpireAfter(
    timeSelector: _ => TimeSpan.FromSeconds(1),
    scheduler: null);

Since this is a breaking change with low impact, I would propose NOT merging this until we already have a major-version change queued up, due to other issues or enhancements.

[dwcullop]

Even though it's a breaking change, I think this is a good thing. Waiting until a major release mitigates the breaking change somewhat... Thanks for this.

Maybe we should examine the APIs for similar ambiguous overloads and put all the fixes into a feature branch that can be merged before the next major rev. That would be better than making more breaking changes later.

[JakenVeina]

Turns out the list version of ExpireAfter has the exact same issue, so I added the fix for that as well.

[RolandPheasant]

@JakenVeina apart form merge conflicts, is this ready?

[github-actions[bot]]

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