Added a test that shows how two selects with different keys throw a KeyNotFoundException

Showcase for #183

So this isn't a really good unit test, but more of a showcase for the behavior, I also had no idea how I should name the method. 

It just shows that two selects with different keys throw a KeyNotFoundException at line https://github.com/akavache/Akavache/blob/f31a508214003ec51f0f8a788af99cfde6fc8465/Akavache.Sqlite3/OperationQueueCoalescing.cs#L193, which is problematic for `GetCreatedAt`, since it expects an empty enumerable as return value.

I also had to introduce the `ReplaceOperationQueue` method, so I could setup a cache with an existing queue, feel free to remove it, I haven't found a cleaner way to do this.