Improve performance for RelationalCommandCache by adding a hashcode to the Command

[ADNewsom09]

We found in our application that due to a large number of generated items RelationalCommandCache reached capacity the system would need to do linear searches through MemoryCache's ConcurrentDictionary of CommandCacheKeys due to them all being in the same bucket since there is a hard-coded GetHashCode() of 0. This avoids the advantages of ConcurrentDictionary's O(1) lookup time.

I thought that a simple improvement would be to use RuntimeHelpers.GetHashCode(_queryExpression) instead of 0. This would improve the distribution of buckets in the HashSet while maintaining the contract with the overridden Equals while avoiding any hashcode calculation explosion that might happen if we considered the _parameterValues that are also used in Equals or if an overridden Expression GetHashCode looked at the items in the Expression. as mentioned in #19859.
Since reference equality is the first thing checking in Equals this will be consistent to uniquely find a CommandCacheKey