Close ID pool when closing client

For every client that `SimpleClientFactory` creates, a goroutine is started for the `idPool`.

When you call `client.Close()` the connection is closed but the `idPool` remains running.

With this change, the `idPool` is properly closed.

Co-authored-by: Adrian Philipp <adri@users.noreply.github.com>

client.go

@@ -67,6 +67,11 @@ type idPool struct {
 	IDs chan uint16
 }
 
+// Close closes the IDs channel
+func (p *idPool) Close() {
+	close(p.IDs)
+}
+
 // AllocID implements Client.AllocID
 func (p *idPool) Alloc() uint16 {
 	return <-p.IDs
@@ -342,6 +347,7 @@ func (c *client) Do(req *Request) (resp *ResponsePipe, err error) {
 // If the inner connection has been closed before,
 // this method would do nothing and return nil
 func (c *client) Close() (err error) {
+	c.ids.Close()
 	if c.conn == nil {
 		return
 	}