IX-Async implementation of ForEachAsync possibly failing to dipose enumerator

[divega]

We got an EF Core 1.0.0 RC2 bug report at dotnet/efcore#5440 that could be explained by ForEachAsync() not timely calling Dispose() on the IAsyncEnumerator<T>.

We didn't get a full repro but the customers confirmed that the issue went away when we replaced IX-Async's implementation of ForEachAsync() with our own.

cc @anpete

[clairernovotny]

Is there any chance you can submit your version as a PR?

[clairernovotny]

This should be fixed in #170. Can you please verify with 3.0.1-build00046 or later?