[ofiwg] [RFC] refining 'variable message' support

[Hefty, Sean]

The following PR introduces a new mode bit, FI_BUFFERED_RECV.
 
https://github.com/ofiwg/libfabric/pull/4108

This is useful for providers that must manage their own network buffers, such as RxM, RxD, and shared memory, and can avoid extra receive side copies.  It's anticipated that the multi-rail provider will use this.

This change refines 'variable message' support.  To recap, variable messages has providers report the message size prior to data delivery and supports transfers larger than the transport protocol can handle.  This avoid multiple layers needing to implement something like a rendezvous protocol.  Variable messages will layer over the FI_BUFFERED_RECV definition.

- Sean