SSHLibrary should not try to "logger.log_background_messages" unless in main thread.

Nobody has claimed this yet.

I am in my keyword running some parallel tasks using concurrent.futures and using robotbackgroundlogger to log the messages. After all threads are finished I run logger.log_background_messages() in the main thread since it only allows running it in main thread.

The problem is that SSHLibrary() automatically tries to call logger.log_background_messages() in it’s close function.

From the client.py [source code](https://github.com/MarketSquare/SSHLibrary/blob/master/src/SSHLibrary/client.py) I can see the following snippet

```
def close(self):
"""Closes the connection."""
if self.tunnel:
self.tunnel.close()
self._sftp_client = None
self._scp_transfer_client = None
self._scp_all_client = None
self._shell = None
self.client.close()
try:
logger.log_background_messages()
except AttributeError:
pass
```

Which means that closing any SSH connection in a thread would raise an error, since you are not allowed to run the function logger.log_background_messages() unless in the main thread.

Something like this could be done,

```
import threading

if threading.current_thread() == threading.main_thread():
try:
logger.log_background_messages()
except AttributeError:
pass
```

Start in src/SSHLibrary/client.py at Client.close and reproduce the issue by closing an SSH connection from a concurrent.futures worker while using robotbackgroundlogger. Verify that background messages are only logged from the main thread, that worker-thread closing no longer raises an error, and that main-thread closing still handles logging.

Written by the indexing model from the issue text.