Add Last db view in kvdb and statedb

Last db view is an opened pebble db which always contains a checkpoint from the
last batch.  Methods to access this last batch are thread safe so that views of
the last checkpoint can be made anywhere and with a consistent view of the
state.

api/api_test.go

@@ -355,6 +355,10 @@ func TestMain(m *testing.M) {
 			panic(err)
 		}
 	}
+	// Make a checkpoint to make the accounts available in Last
+	if err := api.s.MakeCheckpoint(); err != nil {
+		panic(err)
+	}
 
 	// Generate Coordinators and add them to HistoryDB
 	const nCoords = 10