sophiayangDB has created PR #2804: PYTHON-5671: Use fine-grained locks in the connection pool to reduce contention in mongo-python-driver

Issue Text:
PYTHON-5671

1. 1. Changes in this PR
      Based on investigation in PYTHON-5007, using fine-grained locks improved thread efficiency by about 20% in a small read scenario. This PR seeks to implement the suggestions.

Previously, code in `pool.py` was protected by the sole lock `self.lock` and its two conditionals. Now, there are six individual locks that are the sole protectors of the following:

• `self._conns_lock`
  Protects `self.conns`
• `self._active_contexts_lock`
  Protects `self.active_contexts` as well as the subsequent updates to `self.__pinned_sockets`, `self.active_sockets`, `next_connection_id`, `self.ncursors`, and `self.ntxns`
• `self.size_cond`
  Protects `self.requests` and `self.gen`
• `self._max_connecting_cond`
  Protects `self._pending`
• `self._operation_count_lock`
  Protects `self.operation_count`
• `self.lock`
  Protects `self.state`

Other strategies used to mitigate lock contention include:

• Moving lines that do not require a lock to be performed before lock acquisition
• Use of temporary, local variables to avoid nested lock contention
• Removing lock acquisition from `update_is_writable()` as its sole caller already has the lock
• Copying `self.active_contexts` to update its references when additional additions/deletions to the set should not be operated upon or their absence does not modify behavior, avoids errors that arise if calling on the original and it's length/size changes

1. 1. Test Plan
      Tests already exist in `main/test/test_pooling.py` and `test/asynchronous/test_pooling.py`. Since this was only a code refactor, no additional tests were added.

1. 1. Checklist
      <!-- Do not delete the items provided on this checklist. -->

1. 1. 1. Checklist for Author

• [ ] Did you update the changelog (if necessary)?
• [yes] Is there test coverage?
• [n/a] Is any followup work tracked in a JIRA ticket? If so, add link(s).

1. 1. 1. Checklist for Reviewer

• [ ] Does the title of the PR reference a JIRA Ticket?
• [ ] Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• [ ] Is all relevant documentation (README or docstring) updated?