Don't leak sendIndexes on disconnect (fixes #2589)

Adds a Closed() method on protocol.Connection and clears up
wireformatConnection a little too.

lib/model/model_test.go

@@ -243,6 +243,10 @@ func (FakeConnection) Ping() bool {
 	return true
 }
 
+func (FakeConnection) Closed() bool {
+	return false
+}
+
 func (FakeConnection) Statistics() protocol.Statistics {
 	return protocol.Statistics{}
 }