PYTHON-4780 Implement fast path for server selection with Primary

[blink1073]

Tested with the following script:

from pymongo import MongoClient
from pymongo.read_preferences import ReadPreference
from concurrent.futures import ThreadPoolExecutor, wait

client = MongoClient()
session = client.start_session()
read_preference = ReadPreference.PRIMARY


def test():
    for i in range(100):
        client._select_server(read_preference, session, "testOperation")



def test2_target():
    client._select_server(read_preference, session, "testOperation")


def test2():
    futures = []
    with ThreadPoolExecutor(max_workers=100) as pool:
        for i in range(100):
            futures.append(pool.submit(test2_target))
        wait(futures)
    

if __name__ == '__main__':
    import timeit
    print(timeit.timeit("test()", setup="from __main__ import test, client, read_preference, session", number=1000))
    print(timeit.timeit("test2()", setup="from __main__ import test2, client, read_preference, session", number=1000))

On master:
0.4806627919897437
1.5110677920019953

With this change:
0.17988054199668113
1.233099208009662

[blink1073]

I didn't schedule a perf build because our perf tests only run on standalone.

[ShaneHarvey]

Minor note for the benchmark, thread dispatching could be dominating test2. Could you change it to pool.submit(test) and just remove test2_target? That would probably show a larger perf increase for threaded.

[ShaneHarvey]

should this block also have a fast path for returning [] when there is no primary?