Mark sender and timer threads as fork safe for Puma

[joshuay03]

We're in the process of enabling Puma's preload_app! config at @buildkite. When auditing threads spawned during boot, Puma flagged this gem's sender thread as potentially not fork safe. However, based on the documentation for this gem and the implementation of Datadog::Statsd::Sender#add, the sender thread appears fork safe to me. The same reasoning applies to the Statsd Timer thread used when buffer_flush_interval is configured, since Sender#add also restarts the flush timer after a fork.

This PR adds a :fork_safe thread variable to both threads, which Puma uses to exclude them from its safety reporting, so that anyone else auditing their threads this way won't see false positives from this gem.

Refs:

• How Puma uses the variable: https://github.com/puma/puma/blob/97c7d129a940c809fb379b3ecf314d39a18a332b/lib/puma/cluster.rb#L370-L379
• How Rails sets the variable: https://github.com/rails/rails/blob/8bf7a3d4d5760da094031681d354a4e148c2ded6/activerecord/lib/active_record/connection_adapters/abstract/connection_pool/reaper.rb#L42-L44

[joshuay03]

Bump 🙏🏽

[carlosroman]

@codex review

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[carlosroman]

@joshuay03 Let me fix the CI so that we can run the tests for this PR. Sorry about that.