[cluster] keep track of node counts cluster-wide.

This generalizes exchange of signals between the ranks using a non-blocking all-reduce. It is now used for the stop signal and the node count, but should be easily generalizable (TB hits, and ponder still missing). It avoids having long-lived outstanding non-blocking collectives (removes an early posted Ibarrier). A bit too short a test, but not worse than before:

Score of new-r4-1t vs old-r4-1t: 459 - 401 - 1505  [0.512] 2365
Elo difference: 8.52 +/- 8.43

src/Makefile

@@ -359,7 +359,7 @@ endif
 ### 3.10 MPI
 ifneq (,$(findstring mpi, $(CXX)))
 	mpi = yes
-	CXXFLAGS += -DUSE_MPI -Wno-cast-qual
+	CXXFLAGS += -DUSE_MPI -Wno-cast-qual -fexceptions
         DEPENDFLAGS += -DUSE_MPI
 endif