Add experimental static size mode for ofi transport

Whenever --ofi-size-hint > 0, the ofi transport does not use the control
band. Multipart is not supported.

fairmq/ofi/Context.cxx

@@ -37,6 +37,7 @@ Context::Context(FairMQTransportFactory& sendFactory,
     : fIoWork(fIoContext)
     , fReceiveFactory(receiveFactory)
     , fSendFactory(sendFactory)
+    , fSizeHint(0)
 {
     InitThreadPool(numberIoThreads);
 }