mgr/volumes: a lock to guard against races reading/writing config

mgr/volumes: a lock to guard against races reading/writing config

Fixes: https://tracker.ceph.com/issues/55583
Use a python threading lock to avoid race conditions where the
config file is being both read and written to at the same time.

Before this change, the content of the config file being parsed could be
'corrupted' by the MetadataManager racing with itself.  Along with the
previous two patches, additional logging was added to the mgr code to
produce the simplified version of the mgr log below:

```
[volumes INFO volumes.fs.operations.versions.metadata_manager] READ: b'[GLOBAL]\nversion = 2\ntype = clone\npath = /volumes/Park/babydino2/c9f773af-5221-49c6-846c-d65c0920ae3f\nstate = pending\n\n[source]\nvolume = cephfs\ngroup = Park\nsubvolume = Jurrasic\nsnapshot = dinodna0\n\n'
[volumes INFO volumes.fs.operations.versions.metadata_manager] READ: b''
[volumes INFO volumes.fs.operations.versions.metadata_manager] READ: b'[GLOBAL]\nversion = 2\ntype = clone\npath = /volumes/Park/babydino2/c9f773af-5221-49c6-846c-d65c0920ae3f\nstate = pending\n\n[source]\nvolume = cephfs\ngroup = Park\nsubvolume = Jurrasic\nsnapshot = dinodna0\n\n'
[volumes INFO volumes.fs.operations.versions.metadata_manager] wrote 203 bytes to config b'/volumes/Park/babydino2/.meta'
[volumes INFO volumes.fs.operations.versions.metadata_manager] READ: b'a0\n\n'
[volumes INFO volumes.fs.operations.versions.metadata_manager] READ: b''
[volumes ERROR volumes.module] Failed _cmd_fs_clone_cancel(clone_name:babydino2, format:json, group_name:Park, prefix:fs clone cancel, vol_name:cephfs) < "":
Traceback (most recent call last):
  ...
  File "/usr/lib64/python3.6/configparser.py", line 1111, in _read
    raise e
configparser.ParsingError: Source contains parsing errors: b'/volumes/Park/babydino2/.meta'
	[line 13]: 'a0\n'
```

Looking at the above you can see that the log indicates a write to the
config file (of 203 bytes). This happens before the file has finished
reading and thus instead of getting an empty string indicating EOF, it
gets that last four bytes of the new content of the file. The lock
prevents the MetadataManager from both reading and writing the config
file at the same time.

Signed-off-by: John Mulligan <jmulligan@redhat.com>
(cherry picked from commit 2fe83d72d94d1e0939d390554a833cd93b4dd965)