Fixes https://github.com/spring-projects/spring-integration/issues/8708
According to the `org.apache.sshd.common.channel.ChannelAsyncOutputStream.writeBuffer()` JavaDocs cannot be used concurrently.
* Introduce internal `DefaultSftpSessionFactory.ConcurrentSftpClient` extension
of the `DefaultSftpClient` to set a `Lock` around `super.send(cmd, buffer);`
* Remove lock from the `SftpSession` since it now is managed by the mentioned `ConcurrentSftpClient`
**Cherry-pick to `6.1.x` & `6.0.x`**
# Conflicts:
# spring-integration-sftp/src/main/java/org/springframework/integration/sftp/session/SftpSession.java
698a7c36b5