MPI_Waitany broken

[sjeaugey]

All MPI_Waitany are now failing on master on NVIDIA MTT.

Seems like a consequence of the request refactoring.

[sjeaugey]

This is not limited to ib or CUDA, it also fails with tcp.

[jsquyres]

See some Cisco MTT fails for the Waitany tests: https://mtt.open-mpi.org/index.php?do_redir=2331

[sjeaugey]

Fixed by #1766.