feat(jstzd): implement shutdown signal in jstzd server

[huancheng-trili]

Context

Part of JSTZ-168.

JSTZ-168

Description

Implement shutdown signal in jstzd server.

If a program spins up jstzd server and keeps it running, it needs a way to know when jstzd server is terminated, e.g. after someone calls the shutdown endpoint. Otherwise, the program would just hang there not knowing when it can exit. This PR adds a wait method to achieve that. When wait finally returns, it means that jstzd server is shut down.

Manually testing the PR

• Integration test: updated jstzd test to verify that wait actually works

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 47.63%. Comparing base (646ba92) to head (54ce88f).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/jstzd/src/task/jstzd.rs	84.61%	0 Missing and 2 partials ⚠️

Files with missing lines	Coverage Δ
crates/jstzd/src/task/jstzd.rs	73.46% <84.61%> (+0.19%)	⬆️

... and 1 file with indirect coverage changes

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[ryutamago]

🚀

[huancheng-trili]

Offline discussion: channel population is postponed to run being called to prevent the following situation from happening:

let server = JstzdServer::new(...);
server.wait().await;   // <- waits indefinitely because the server is not yet running and thus there's no way to trigger the signal sender

Since the whole run method is not atomic, technically speaking, it's possible that something happens between spawning the server and setting the channel and prevents things from running as expected. This should be fine for now since JstzdServer is not clone-able and thus it's not possible to interrupt run asynchronously.