Locality aware gather (#198)

* first version at optimised scatter

* add test for receiver non local leader

* re-factor and organise the hosts to ranks mappings

* adding double colons for global namespaces

* inline set hosts for ranks and re-factor gather to reduce complexity with more explainatory comments

* add test to check the number of messages sent during gather, and the size of these messages

include/faabric/scheduler/MpiWorld.h

@@ -224,15 +224,16 @@ class MpiWorld
     /* MPI internal messaging layer */
 
     // Track at which host each rank lives
-    std::vector<std::string> rankHosts;
+    std::vector<std::string> hostForRank;
     int getIndexForRanks(int sendRank, int recvRank);
-    std::vector<int> getRanksForHost(const std::string& host);
 
-    // Track ranks that are local to this world, and local/remote leaders
+    // Store the ranks that live in each host
+    std::map<std::string, std::vector<int>> ranksForHost;
+
+    // Track local and remote leaders. The leader is stored in the first
+    // position of the host to ranks map.
     // MPITOPTP - this information exists in the broker
     int localLeader = -1;
-    std::vector<int> localRanks;
-    std::vector<int> remoteLeaders;
     void initLocalRemoteLeaders();
 
     // In-memory queues for local messaging