Change DispatchQueue::dispatch to use notify_one. (The-OpenROAD-Project#308)

This prevents the thundering herd problem and should increase the
scalability of the DispatchQueue significantly.

Additionally the code the DispatchQueue was taken from made this
improvement five years ago:
embeddedartistry/embedded-resources@79ad8a5

Signed-off-by: Drew Lewis <cannada@google.com>

Author: calewis

util/DispatchQueue.cc

@@ -66,7 +66,7 @@ DispatchQueue::dispatch(const fp_t& op)
   // Manual unlocking is done before notifying, to avoid waking up
   // the waiting thread only to block again (see notify_one for details)
   lock.unlock();
-  cv_.notify_all();
+  cv_.notify_one();
 }
 
 void
@@ -79,7 +79,7 @@ DispatchQueue::dispatch(fp_t&& op)
   // Manual unlocking is done before notifying, to avoid waking up
   // the waiting thread only to block again (see notify_one for details)
   lock.unlock();
-  cv_.notify_all();
+  cv_.notify_one();
 }
 
 void