Use multi-piece queue mechanism to avoid unnecessary copying during collectives

[mrzv]

Large queues (> 2GB) are broken into smaller pieces (tagged with tags::piece) to work around MPI limitations. detail::VectorWindow<char> is used to put subsets of the MemoryBuffers into MPI i[s]send routines without having to make a copy. shared_ptrs are used to keep track of the original buffer as long as some in-flight message needs it.

We could use the same mechanism during, e.g., all-to-all operations to avoid making copies between buffers.