IX-Async implementation of GroupJoin possibly leading to deadlock

[divega]

Heads up that this issue was reported on Entity Framework Core 1.0.0 RC2 at dotnet/efcore#5481.

Customer provided a repro at https://github.com/iberodev/EntityFramework5481.

We haven't tried to produce a self-contained repro with only IX-Async in the picture but we worked around it by creating our own implementation of GroupJoin().

cc @anpete

[clairernovotny]

Is there any chance you can submit your implementation of GroupJoin as a PR here?

[anpete]

@onovotny We rely on async/await, which, so far, IX-Async does not want to depend on.

[clairernovotny]

Do you recall the reason why Ix-Async has avoided async/await?

[anpete]

I think it was compat. Not sure if it still matters.

[clairernovotny]

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