Abort compile tasks and associated subprocesses when a client disconnects

[trxcllnt]

If an sccache client disconnects from the server before the request completes, abort the tokio tasks spawned to handle the request. Uses tokio::process::Command::kill_on_drop() to ensure the task's pending subprocesses are also killed.

I've added an extra level of nesting and allowed clippy::redundant_async_block to aid in review, otherwise the diff is dominated by whitespace changes related to un-indenting the body of start_compile_task. I can push a new commit with the redundant async block removed once this PR is approved.

Fixes #2759

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.55556% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.15%. Comparing base (68cd330) to head (27a7542).

Files with missing lines	Patch %	Lines
src/server.rs	74.07%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2761      +/-   ##
==========================================
- Coverage   74.18%   74.15%   -0.03%     
==========================================
  Files          71       71              
  Lines       40491    40537      +46     
==========================================
+ Hits        30037    30062      +25     
- Misses      10454    10475      +21     

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