Suspicious shutdown code

[eschnett]

@amitmurthy points out that these lines

    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.