Delete old checkpoints in stateDB automatically

Introduce a constructor parameter for the StateDB called `keep`, which tells
how many checkpoints to keep.  When doing a new checkpoint, if the number of
existing checkpoints exeeds `keep`, the oldest ones will be deleted.

api/api_test.go

@@ -212,7 +212,7 @@ func TestMain(m *testing.M) {
 		}
 	}()
 	chainID := uint16(0)
-	sdb, err := statedb.NewStateDB(dir, statedb.TypeTxSelector, 0, chainID)
+	sdb, err := statedb.NewStateDB(dir, 128, statedb.TypeTxSelector, 0, chainID)
 	if err != nil {
 		panic(err)
 	}