Implement load balancing

* Implement load balancing

* Implement failover for singleServer with failover

* Add property loading for `acquireHostList` and `loadBalancingStrategy`

* Add load balancing docs

* snapshot version 4.3.0-SNAPSHOT

* Fix bug random host handler

* Fix test properties

* Implement following redirects

* Load balancing/ fallback refactoring

* Change redirect to response code 503 and header 'x-arango-endpoint'

* Fix acquireHostList db call

* Fix changelog

Author: mpv1989

ChangeLog

@@ -1,3 +1,8 @@
+v4.3.0 (2017-11-23)
+---------------------------
+* added load balancing (ArangoDB.Builder.loadBalancingStrategy())
+* added automatic acquiring of hosts for load balancing or as fallback (ArangoDB.Builder.acquireHostList())
+
 v4.2.7 (2017-11-03)
 ---------------------------
 * added ArangoGraph.exists()