New Account and AccountStore API.

This change better organizes account handling by creating a new
AccountStore type and accountstore global variable, with receiver
funcs for all operations that require all accounts.  More Account
funcs are also added to clean up account handling in the RPC code.

Intial work on this done by dhill.

disksync.go

@@ -69,8 +69,8 @@ func (w *Account) writeDirtyToDisk() error {
 	// for validity, and moved to replace the main file.
 	timeStr := fmt.Sprintf("%v", time.Now().Unix())
 
-	adir := accountdir(cfg, w.name)
-	if err := checkCreateAccountDir(adir); err != nil {
+	adir := w.accountdir(cfg)
+	if err := w.checkCreateAccountDir(adir); err != nil {
 		return err
 	}