Improve forking support

[vickenty]

What does this PR do?

Use register_at_fork added in Python 3.7+ to transparently handle process fork when the client is using background threads.

Description of the Change

Forking support is introduced in several layers:

• Per-instance hooks that can be called before and after fork() to disable and re-enable background threads and also flushes pending metrics.
• A weakset is added that tracks all instances of DogStatsd (an opt-out is provided in case unforeseen complications arise).
• Package level hooks that invoke fork hooks on all tracked instances.
• On Python 3.7+, the package level hooks are automatically registered with the Python runtime via os.register_at_fork.

This approach is similar to one used in dd-trace-py for handling forks.

Alternate Designs

Possible Drawbacks

Verification Process

Additional Notes

Release Notes

Review checklist (to be filled by reviewers)

• [ ] Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• [ ] PR title must be written as a CHANGELOG entry (see why)
• [ ] Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• [ ] PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• [ ] PR should not have do-not-merge/ label attached.
• [ ] If Applicable, issue must have kind/ and severity/ labels attached at least.