Move to mutable state

Raft/Measures.fs

@@ -0,0 +1,11 @@
+namespace Raft
+
+/// LogIndex is indexed from 1. We use 0 to indicate "before any history has started".
+[<Measure>]
+type LogIndex
+
+[<Measure>]
+type Term
+
+[<Measure>]
+type ServerId