Rework GetLatestBlockHeight function of the Electrum client

So far, `GetLatestBlockHeight` function used `SubscribeHeaders` under the hood.
That caused a memory leak because `GetLatestBlockHeight` was not interested
in reading from the returned `headersChan` channel. Each call to
`GetLatestBlockHeight` produced a new dangling goroutine blocked on a buffered
channel holding one item

Here we replace `SubscribeHeaders` with `SubscribeHeadersSingle` which
does not create a goroutine supposed to handle future headers notifications.
The `SubscribeHeadersSingle` just return the current chain tip and ignores
further notifications coming from the Electrum server.

See keep-network/go-electrum#5 for further reference.

pkg/bitcoin/electrum/electrum.go

@@ -263,11 +263,11 @@ func (c *Connection) GetLatestBlockHeight() (uint, error) {
 	blockHeight, err := requestWithRetry(
 		c,
 		func(ctx context.Context, client *electrum.Client) (int32, error) {
-			headersChan, err := client.SubscribeHeaders(ctx)
+			tip, err := client.SubscribeHeadersSingle(ctx)
 			if err != nil {
 				return 0, fmt.Errorf("failed to get the blocks tip height: [%w]", err)
 			}
-			tip := <-headersChan
+
 			return tip.Height, nil
 		},
 		"SubscribeHeaders",