Add and use Mprobe, Mrecv, Improbe, Imrecv

[bn222]

If probe is used to find the size of a message followed by a recv, MPI does not
guarantee that the two calls refer to the same message. On larger systems, with
many ranks communicating, different message sizes, using probe+recv results in
race conditions. Therefore, MPI introduced the Mprobe (returning a handle) which
can be passed to Mrecv to receive the same message.

[eschnett]

It seems that MPI_Mprobe was introduced in MPI 3. If this function is used prominently, then we need to update the required version of MPI everywhere.

MPI guarantees that messages using the same communicator, originating from the same source, and having the same tag arrive in order (in the order in which they were sent). When MPI.jl calls MPI_Probe, it extracts source and tag, and uses these when calling MPI_Recv. This should ensure that the same message is matched. Can you point to a counterexample, or a relevant discussion somewhere that describes how MPI_Mprobe is necessary in this case?

[bn222]

You're right, passing back the tag and source ensures that the right message is delivered, and therefor this patch isn't needed. Would it still be useful to have Mprobe and Mrecv exposed through MPI.jl for the compatible versions?

[eschnett]

Definitively! These functions should be added. You'd need to make sure to check the MPI standard version, though – I don't recall how this is done, but there should be other example for this.

[simonbyrne]

Superseded by #699