Slowdown in scheduler after extended eager api usage

This piece of code (if it doesn't hang) slows down significantly at ~1800 iterations for me.
From observing the threads usage it seems like the main thread is constantly occupied and the time period between each spawn increases every iteration (it can be observed through cpu usage on threads).

Most likely related to the huge number of thunks generated in this code over time

```julia
julia> using Dagger

julia> c = Dagger.@spawn 10+10; b = (x) -> x + 10; a = (x) -> x .+ fetch.([Dagger.spawn(b, x + i) for i in 1:100]);

julia> for i in 1:10000
r = Dagger.@spawn a(c)
fetch(r); println(i)
end
```