lnd: ensure that we set the wallet birthday for --noencryptwallet

In this commit, we ensure that we always set the wallet birthday. If the
user has provided a seed, or is creating a new one, then it will be
overwritten below. However, before this commit, if a user started with
the --noencryptwallet flag, then we would _always_ start to rescan from
genesis with the recent bug fix to ensure that we always start after the
birthday.

lnd.go

@@ -204,7 +204,7 @@ func lndMain() error {
 	var (
 		privateWalletPw = lnwallet.DefaultPrivatePassphrase
 		publicWalletPw  = lnwallet.DefaultPublicPassphrase
-		birthday        time.Time
+		birthday        = time.Now()
 		recoveryWindow  uint32
 		unlockedWallet  *wallet.Wallet
 	)