sweep: rename methods for clarity

We now rename "third party" to "unknown" as the inputs can be spent via
an older sweeping tx, a third party (anchor), or a remote party (pin).
In fee bumper we don't have the info to distinguish the above cases, and
leave them to be further handled by the sweeper as it has more context.

sweep/fee_bumper_test.go

@@ -1597,7 +1597,7 @@ func TestProcessRecordsInitialSpent(t *testing.T) {
 		require.Nil(t, result.ReplacedTx)
 
 		// The error should be set.
-		require.ErrorIs(t, result.Err, ErrThirdPartySpent)
+		require.ErrorIs(t, result.Err, ErrUnknownSpent)
 		require.Equal(t, requestID, result.requestID)
 	}
 }
@@ -1786,7 +1786,7 @@ func TestProcessRecordsSpent(t *testing.T) {
 		require.Equal(t, tx, result.Tx)
 
 		// No error should be set.
-		require.ErrorIs(t, result.Err, ErrThirdPartySpent)
+		require.ErrorIs(t, result.Err, ErrUnknownSpent)
 		require.Equal(t, requestID, result.requestID)
 	}
 }