impl(pubsub): lease loop

[dbolduc]

Part of the work for #3957

Add a lease loop. The loop runs on end, performing any of the following actions:

• handle a shutdown
• flush acks/nacks
• extend leases
• accept a new message
• handle incoming acks/nacks

I combined these things into a struct, but I may change my mind later.

On testing...

TIL: mock.checkpoint(). It means check all the expectations at this exact moment.

I found it easiest to use an Arc<Mutex<Mock>> which allowed me to give the mock to the lease loop AND set expectations on the mock outside of the lease loop.

I also tried to recover the time at which events happen from within the mock expectations, but decided that was more complicated.

Because tokio::test is single threaded (if you want to start_paused = true), I found that I needed to yield the current execution so tokio would actually launch the lease loop. Otherwise it would not launch the lease loop until the first .await point, and the timing was all messed up.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.77%. Comparing base (97470cb) to head (75ba458).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pubsub/src/subscriber/lease_loop.rs	87.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4120      +/-   ##
==========================================
- Coverage   94.81%   94.77%   -0.04%     
==========================================
  Files         182      183       +1     
  Lines        6887     6911      +24     
==========================================
+ Hits         6530     6550      +20     
- Misses        357      361       +4     

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[suzmue]

nit: all these comments should end with periods.

Comments should usually be complete sentences. Start with a capital letter, end with a period (.). - https://doc.rust-lang.org/stable/style-guide/#comments

[dbolduc]

Fixed