swap's block size made into a compile-time parameter

src/config.hpp

@@ -62,6 +62,10 @@ namespace zmq
         //  Maximal delta between high and low watermark.
         max_wm_delta = 1024,
 
+        //  Swap inteligently batches data for writing to disk. The size of
+        //  the batch in bytes is specified by this option.
+        swap_block_size = 8192,
+
         //  Maximum number of events the I/O thread can process in one go.
         max_io_events = 256,