master_slave DArray copy

Hello,

First, thank you for a great package!

I have had an issue with `copy` of `DArray` when connected in `master_slave` topology.

```julia
julia> addprocs(2,topology=:master_slave)
2-element Array{Int64,1}:
2
3

julia> nprocs()
3
julia> using DistributedArrays

julia> dd=drand((11,21,2,13),workers(),[1,1,1,2]);

julia> @eval @everywhere println(localindexes($dd))
(1:0,1:0,1:0,1:0)
From worker 2: (1:11,1:21,1:2,1:7)
From worker 3: (1:11,1:21,1:2,8:13)

julia> ddc=copy(dd);
ERROR: On worker 2:
peer 3 is not connected to 2. Topology : master_slave
...
```
I have shortened the error output, surely you can reproduce it.

Is there a reason why this is an error?

Especially that this works...
```julia

julia> dd=drand((11,21,2,13),workers(),[1,1,1,2]);

julia> @everywhere using DistributedArrays

julia> @eval ddc2=DArray(I->copy(localpart($dd)),$dd);

julia> sumabs2(ddc2-dd)
0.0

julia> @eval @everywhere println(localindexes($ddc2))
(1:0,1:0,1:0,1:0)
From worker 2: (1:11,1:21,1:2,1:7)
From worker 3: (1:11,1:21,1:2,8:13)

julia> @eval @everywhere println(localindexes($dd))
(1:0,1:0,1:0,1:0)
From worker 3: (1:11,1:21,1:2,8:13)
From worker 2: (1:11,1:21,1:2,1:7)
```
Finally, is there a reason why `similar` does not yield a `DArray` with the same distribution?

```julia
julia> dd=drand((11,21,2,13),workers(),[1,1,1,2]);

julia> @eval @everywhere println(localindexes($dd))
(1:0,1:0,1:0,1:0)
From worker 2: (1:11,1:21,1:2,1:7)
From worker 3: (1:11,1:21,1:2,8:13)

julia> ddc=similar(dd);

julia> @eval @everywhere println(localindexes($ddc))
(1:0,1:0,1:0,1:0)
From worker 2: (1:11,1:11,1:2,1:13)
From worker 3: (1:11,12:21,1:2,1:13)
```

I appreciate any help on this, and again thanks for the effort with this package!

Start by reproducing the master_slave examples using DArray, copy, similar, and localpart as shown in the issue. Inspect the implementations and tests for DArray copy and similar, then verify behavior with the provided localindexes and sumabs2 checks. Done means copy and similar handle the reported distribution or clearly document the intended behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.