HG: internal bulk transfer and one-way RPCs

[soumagne]

When going over the eager message size limit, Mercury automatically uses an overflow mechanism that lets larger messages be transferred using RMA. When using one-way RPCs however, there is currently no mechanism that lets the user buffers remain valid until those transfers are over. In that particular case, oneway RPCs should implicitly be made two-way, letting the bulk transfer callback send a notification back, which would mark the RPC as complete.