cmd/geth, cmd/utils, core, rpc: renamed to blockchain

* Renamed ChainManager to BlockChain
* Checkpointing is no longer required and never really properly worked
when the state was corrupted.

cmd/geth/js_test.go

@@ -196,7 +196,7 @@ func TestBlockChain(t *testing.T) {
 	tmpfile := filepath.Join(extmp, "export.chain")
 	tmpfileq := strconv.Quote(tmpfile)
 
-	ethereum.ChainManager().Reset()
+	ethereum.BlockChain().Reset()
 
 	checkEvalJSON(t, repl, `admin.exportChain(`+tmpfileq+`)`, `true`)
 	if _, err := os.Stat(tmpfile); err != nil {