[services] Rust Integration - c++ - Implement worker in write reactor

Summary:
Depends on D5070

In response to this issue, we should implement the code that will delegate the work away from the reactors to separate threads.

Here, I implement using the worker in the write reactors.

Test Plan: Services build as usual.

Reviewers: jon, tomek, max

Reviewed By: tomek

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5071