Mark sender thread 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.

This PR adds a :fork_safe thread variable that Puma uses to exclude threads 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