Suspicious shutdown code

@amitmurthy points out that these lines

``` Julia
for i in 1:num_send_loops
fetch(mgr.sending_done)
end
```

in `receive_event_loop` of `cman.jl` look suspicious, as they read the same value multiple times (`fetch` is a non-destructive read). This loop should likely use `take!` instead.

See also the respective in-lin discussion in https://github.com/JuliaParallel/MPI.jl/pull/105/files/20cee8dc639e87db6115de4ce32f4b7376935fdb#r65022731.