channeldb+invoices: fix what pending invoice means in ChannelDB

This commits builds on top of PR #3694 to further clarify invoice
state by defining pending invoices as the ones which are not
settled or canceled. Automatic cancellation of expired invoices
makes this possbile. While this change only directly affects
ChannelDB, users of the listinvoices RPC will receive actual
pending invoices when pending_only flag is set.

lnrpc/rpc.pb.go

@@ -7556,7 +7556,9 @@ func (m *PaymentHash) GetRHash() []byte {
 }
 
 type ListInvoiceRequest struct {
-	/// If set, only unsettled invoices will be returned in the response.
+	//*
+	//If set, only invoices that are not settled and not canceled will be returned
+	//in the response.
 	PendingOnly bool `protobuf:"varint,1,opt,name=pending_only,proto3" json:"pending_only,omitempty"`
 	//*
 	//The index of an invoice that will be used as either the start or end of a

lnrpc/rpc.proto

@@ -2450,7 +2450,10 @@ message PaymentHash {
 }
 
 message ListInvoiceRequest {
-    /// If set, only unsettled invoices will be returned in the response.
+    /**
+    If set, only invoices that are not settled and not canceled will be returned
+    in the response.
+    */
     bool pending_only = 1 [json_name = "pending_only"];
 
     /**

lnrpc/rpc.swagger.json

@@ -896,7 +896,7 @@
         "parameters": [
           {
             "name": "pending_only",
-            "description": "/ If set, only unsettled invoices will be returned in the response.",
+            "description": "*\nIf set, only invoices that are not settled and not canceled will be returned\nin the response.",
             "in": "query",
             "required": false,
             "type": "boolean",