Set LWM to half of HWL.

This reduces chances of race between writer deactivation and activation.

Reader sends activation command to writer when number or messages is
multiple of LWM. In situation with high throughput (millions of messages
per second) and correspondingly large HWM (e.g. 10M) the difference
between HWM needs to be large enough - so that activation command is
received before pipe becomes full.

src/pipe.cpp

@@ -460,14 +460,9 @@ int zmq::pipe_t::compute_lwm (int hwm_)
     //     result in low performance.
     //
     //  Given the 3. it would be good to keep HWM and LWM as far apart as
-    //  possible to reduce the thread switching overhead to almost zero,
-    //  say HWM-LWM should be max_wm_delta.
-    //
-    //  That done, we still we have to account for the cases where
-    //  HWM < max_wm_delta thus driving LWM to negative numbers.
-    //  Let's make LWM 1/2 of HWM in such cases.
-    int result = (hwm_ > max_wm_delta * 2) ?
-        hwm_ - max_wm_delta : (hwm_ + 1) / 2;
+    //  possible to reduce the thread switching overhead to almost zero.
+    //  Let's make LWM 1/2 of HWM.
+    int result = (hwm_ + 1) / 2;
 
     return result;
 }