refactor vservers map to RingMap to be able to index it

- in preparation for periodical rebalancing
- makes it possible to look up the next vserver for iterating through
  it, after refreshing the map in-between
- added some necessary RingMap functions

test/FediChordSpec.hs

@@ -19,6 +19,7 @@ import           Hash2Pub.ASN1Coding
 import           Hash2Pub.DHTProtocol
 import           Hash2Pub.FediChord
 import           Hash2Pub.FediChordTypes
+import           Hash2Pub.RingMap
 
 spec :: Spec
 spec = do
@@ -304,7 +305,7 @@ exampleNodeState = RemoteNodeState {
 exampleLocalNode :: IO (LocalNodeState MockService)
 exampleLocalNode = do
     realNodeSTM <- newTVarIO $ RealNode {
-            vservers = HMap.empty
+            vservers = emptyRMap
           , nodeConfig = exampleFediConf
           , bootstrapNodes = confBootstrapNodes exampleFediConf
           , nodeService = MockService