Wrapping request client calls with a read/write mutex

It happens that keep-core client panics when interacting with the
go-electrum library. We noticed "concurrent write to websocket connection" error
was thrown when calling GetLatestBlockHeight() function. The stack trace
leads to gorilla/websocket/WriteMessage which is called from the
go-electrum client. The latest block height call was already wrapped
with a read mutex, but we should also wrap it with the write mutex as
well to prevent such concurrent errors.

pkg/bitcoin/electrum/electrum.go

@@ -771,9 +771,9 @@ func requestWithRetry[K interface{}](
 			requestCtx, requestCancel := context.WithTimeout(ctx, c.config.RequestTimeout)
 			defer requestCancel()
 
-			c.clientMutex.RLock()
+			c.clientMutex.Lock()
 			r, err := requestFn(requestCtx, c.client)
-			c.clientMutex.RUnlock()
+			c.clientMutex.Unlock()
 
 			if err != nil {
 				return fmt.Errorf("request failed: [%w]", err)