…python GC doesn't kick in (#10809)

_<<Description copied from the [original
PR](https://github.com/DataDog/dd-trace-py/pull/10624)>>_

We (Modal Labs) noticed that enabling tracing in parts of our code
caused the Python GC to kick in a lot and add large delays to our event
loop. A simple repro (see test) showed us that there was a self
reference cycle in span when a top-level span's local root was assigned
to the span itself, preventing Python from cleaning up the objects by
reference counting.

The change itself is pretty straightforward - we use None to represent
when a span's local root is itself. The span still exposes _local_root
with the same behavior via a property, and internally stores a
_local_root_value that holds a non-self-referential span. This also
means that the tracer no longer needs to explicitly set the local root
for top-level spans. This should be fairly non-risky as it doesn't
change any code behavior.

## Checklist
- [x] PR author has checked that all the criteria below are met
- The PR description includes an overview of the change
- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
- Backport labels are set in a manner that is consistent with the
[release branch maintenance
policy](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting)

---------

Co-authored-by: Daniel Shaar <[email protected]>
Co-authored-by: Daniel Shaar <[email protected]>
(cherry picked from commit e160b36)