Add --nocheckpoints option.

This commit provides a new flag, --nocheckpoints, to disable built-in
checkpoints.

Checkpoints are used for a number of things such a ensuring
the block chain being downloaded matches various known good blocks,
allowing quicker verification on old blocks since scripts don't have to be
executed, and preventing forks from old blocks, etc.

blockmanager.go

@@ -730,6 +730,10 @@ func newBlockManager(s *server) (*blockManager, error) {
 		quit:             make(chan bool),
 	}
 	bm.blockChain = btcchain.New(s.db, s.btcnet, bm.handleNotifyMsg)
+	bm.blockChain.DisableCheckpoints(cfg.DisableCheckpoints)
+	if cfg.DisableCheckpoints {
+		log.Info("[BMGR] Checkpoints are disabled")
+	}
 
 	log.Infof("[BMGR] Generating initial block node index.  This may " +
 		"take a while...")

config.go

@@ -59,6 +59,7 @@ type config struct {
 	UseTor             bool          `long:"tor" description:"Specifies the proxy server used is a Tor node"`
 	TestNet3           bool          `long:"testnet" description:"Use the test network"`
 	RegressionTest     bool          `long:"regtest" description:"Use the regression test network"`
+	DisableCheckpoints bool          `long:"nocheckpoints" description:"Disable built-in checkpoints.  Don't do this unless you know what you're doing."`
 	DbType             string        `long:"dbtype" description:"Database backend to use for the Block Chain"`
 	Profile            string        `long:"profile" description:"Enable HTTP profiling on given port -- NOTE port must be between 1024 and 65536"`
 	DebugLevel         string        `short:"d" long:"debuglevel" description:"Logging level {trace, debug, info, warn, error, critical}"`