PYTHON-4482 Make _ServerSessionPool lock-free and still thread safe

[ShaneHarvey]

https://jira.mongodb.org/browse/PYTHON-4482

Still todo:

• Benchmark shows a notable improvement for both single threaded and multi-threaded:

(pymongo-py39) ➜  mongo-python-driver git:(PYTHON-4482) python bench-command.py 1  
Python: 3.9.13, PyMongo: 4.8.0.dev0, MongoDB: 5.0.15
Running 10000 operations across 1 thread(s)
command: 1.32s
find_one: 1.93s
insert_one: 1.84s
(pymongo-py39) ➜  mongo-python-driver git:(PYTHON-4482) python bench-command.py 100
Python: 3.9.13, PyMongo: 4.8.0.dev0, MongoDB: 5.0.15
Running 10000 operations across 100 thread(s)
command: 1.22s
find_one: 1.52s
insert_one: 1.35s

VS master:

(pymongo-py39) ➜  mongo-python-driver git:(master) python bench-command.py 1
Python: 3.9.13, PyMongo: 4.8.0.dev0, MongoDB: 5.0.15
Running 10000 operations across 1 thread(s)
command: 1.36s
find_one: 1.95s
insert_one: 1.90s
(pymongo-py39) ➜  mongo-python-driver git:(master) python bench-command.py 100
Python: 3.9.13, PyMongo: 4.8.0.dev0, MongoDB: 5.0.15
Running 10000 operations across 100 thread(s)
command: 1.64s
find_one: 1.67s
insert_one: 1.49s

[Jibola]

LGTM

[Jibola]

why was session_timeout_minutes removed here?

[ShaneHarvey]

Because session_timeout_minutes was only used to clear stale sessions and it seems redundant to do that on both get_session and return_session. It's good enough to clear stale sessions only when getting a session from the pool.