bad scaling in `map`

```
julia> using Distributed

julia> addprocs(2);

julia> @everywhere using DistributedArrays

julia> a = fill(1000,10);

julia> da = distribute(a);

julia> @time map(x->rand(x,x)^2, a);
0.903241 seconds (63.61 k allocations: 155.698 MiB, 29.39% gc time)

julia> @time map(x->rand(x,x)^2, da);
0.967328 seconds (776.84 k allocations: 38.713 MiB)
```
(first time compilation omitted)

Even though this is embarrassingly parallel, the distributed version is consistently around the same time or slower. I tried this in julia 0.3 and the distributed time is around 0.5 seconds, close to the expected ~2x speedup.