make Request objects immutable

[simonbyrne]

Fixes #643

[vchuravy]

The buffer rooting was kinda nice, since it prevented accidental GC errors

[simonbyrne]

Yeah, that's my main reluctance to change this. Not sure what the best option is.

[simonbyrne]

I think the biggest problem is going to be where we allocate the actual send buffer inside the sender, e.g.:

MPI.jl/src/pointtopoint.jl

Lines 75 to 76 in d34be21

	Isend(data, dest::Integer, tag::Integer, comm::Comm) =
	Isend(Buffer_send(data), dest, tag, comm)

and

MPI.jl/src/pointtopoint.jl

Lines 88 to 91 in d34be21

	function isend(obj, dest::Integer, tag::Integer, comm::Comm)
	buf = MPI.serialize(obj)
	Isend(buf, dest, tag, comm)
	end

I'm not sure there is a good solution to this.

[vchuravy]

Yeah we could have an AbstractRequest and then two subtypes, one of it rooting the other one doesn't.

But generally speaking I prefer the safety of the current approach and I am less worried about the cost of memory allocations for these small objects.

[simonbyrne]

Yeah, I don't see a great way around it, unfortunately.

It's not the cost of the small objects I'm worried about, it's the cost of the allocations inside the Waitall! (but RequestSet should hopefully help with that).

[simonbyrne]

Closed in favor of #677.