close_all_connections() raise RuntimeError when called in a thread

Nobody has claimed this yet.

In my code, SSHLibrary used like below and this code is run by another thread:
```python
SSHLibrary(...).open_connection(...)
# do something here
SSHLibrary.close_all_connections()
```

This raise exception:
```python
File "/p4_ws/doyou89.jung/workspace/projects/cosmos/.venv/lib/python3.8/site-packages/SSHLibrary/library.py", line 765, in close_all_connections
self._connections.close_all()
File "/p4_ws/doyou89.jung/workspace/projects/cosmos/.venv/lib/python3.8/site-packages/SSHLibrary/sshconnectioncache.py", line 27, in close_all
connection.close()
File "/p4_ws/doyou89.jung/workspace/projects/cosmos/.venv/lib/python3.8/site-packages/SSHLibrary/pythonclient.py", line 302, in close
return super(PythonSSHClient, self).close()
File "/p4_ws/doyou89.jung/workspace/projects/cosmos/.venv/lib/python3.8/site-packages/SSHLibrary/abstractclient.py", line 160, in close
logger.log_background_messages()
File "/p4_ws/doyou89.jung/workspace/projects/cosmos/.venv/lib/python3.8/site-packages/robotbackgroundlogger.py", line 101, in log_background_messages
raise RuntimeError(
RuntimeError: Logging background messages is only allowed from the main thread. Current thread name: schedule_commands_01
```

Because close() method in abstractclient.py do not handle RuntimeError, the code of close() is:
```python
def close(self):
"""Closes the connection."""
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
```

I think RuntimeError should be added here.

Start with the close() method in abstractclient.py and trace how close_all_connections() reaches it through sshconnectioncache.py. Reproduce the shown worker-thread sequence and verify that closing the connection no longer raises the RuntimeError from background logging.

Written by the indexing model from the issue text.