block_session: extend interface with sync() call

To support components, which implement the block session's server side
rpc object, and which doesn't write data to their device backend immediately,
an additional synchronization call is needed. Thereby, clients like for
instance a file system can tell these components, when a synchronization is
required.

Ref #113

gems/src/server/http_block/main.cc

@@ -178,6 +178,8 @@ namespace Block {
 				ops->set_operation(Packet_descriptor::READ);
 			}
 
+			void sync() {}
+
 			/**
 			 * Signal indicating that transmit thread is ready
 			 */