Add basic infrastructure for upgrading btcd.

This commit adds a basic infrastructure to allow upgrades to happen to
btcd as needed.  This paves the way for the upcoming data path changes to
be automatically updated for the user as needed and also ensures any
future changes that might require upgrades already have an established
way of performing the needed upgrades.

btcd.go

@@ -112,6 +112,13 @@ func btcdMain() error {
 		loggers = setLogLevel(cfg.DebugLevel)
 	}
 
+	// Perform upgrades to btcd as new versions require it.
+	err = doUpgrades()
+	if err != nil {
+		log.Errorf("%v", err)
+		return err
+	}
+
 	// Load the block database.
 	db, err := loadBlockDB()
 	if err != nil {

upgrade.go

@@ -0,0 +1,10 @@
+// Copyright (c) 2013 Conformal Systems LLC.
+// Use of this source code is governed by an ISC
+// license that can be found in the LICENSE file.
+
+package main
+
+// doUpgrades performs upgrades to btcd as new versions require it.
+func doUpgrades() error {
+	return nil
+}