lnwallet: add new method IsSynced to the WalletController interface

This commit adds a new method to the WalletController interface:
IsSynced. The role of the function is to query the local wallet about
if it thinks it has fully synced to the tip of the current main chain.
This function can be useful within U.I’s to block off certain
functionality until the wallet is fully synced to the main chain.

lnwallet/interface.go

@@ -174,13 +174,17 @@ type WalletController interface {
 	// related to the wallet are seen within the network, or found in
 	// blocks.
 	//
-	// NOTE: a non-nil error shuold be returned if notifications aren't
+	// NOTE: a non-nil error should be returned if notifications aren't
 	// supported.
 	//
 	// TODO(roasbeef): make distinct interface?
 	SubscribeTransactions() (TransactionSubscription, error)
 
-	// Start initializes the wallet, making any neccessary connections,
+	// IsSynced returns a boolean indicating if from the PoV of the wallet,
+	// it has fully synced to the current best block in the main chain.
+	IsSynced() (bool, error)
+
+	// Start initializes the wallet, making any necessary connections,
 	// starting up required goroutines etc.
 	Start() error