Enqueue background jobs in parallel

[Turbo87]

This PR adjusts our BackgroundJob::enqueue() fn to be able to take advantage of Postgres query pipelining and then uses it in various places to reduce the amount of waiting for queries to be finished.

Related:

• worker/jobs/rss: Run test setup queries concurrently #9663
• controllers/summary: Use tokio::try_join!() to perform database queries in parallel #10082
• https://rust-lang.zulipchat.com/#narrow/stream/318791-t-crates-io/topic/postgres.20pipelining

[codecov]

Codecov Report

Attention: Patch coverage is 59.03614% with 34 lines in your changes missing coverage. Please review.

Project coverage is 89.35%. Comparing base (f88a3eb) to head (289be81).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/bin/crates-admin/delete_crate.rs	0.00%	10 Missing ⚠️
src/bin/crates-admin/yank_version.rs	0.00%	8 Missing ⚠️
src/controllers/krate/publish.rs	75.00%	8 Missing ⚠️
src/bin/crates-admin/delete_version.rs	0.00%	7 Missing ⚠️
crates/crates_io_worker/src/background_job.rs	94.11%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10087      +/-   ##
==========================================
- Coverage   89.37%   89.35%   -0.03%     
==========================================
  Files         294      294              
  Lines       31283    31308      +25     
==========================================
+ Hits        27960    27975      +15     
- Misses       3323     3333      +10     

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[LawnGnome]

This LGTM, but I also am not going to pretend to be fully across all the nuances of async Diesel.