feat!: use swansong instead of stopper + clone counter

[jbr]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 58.06452% with 39 lines in your changes are missing coverage. Please review.

Project coverage is 49.94%. Comparing base (15029f8) to head (bc729de).

Files	Patch %	Lines
smol/src/server/unix.rs	47.36%	10 Missing ⚠️
http/src/conn.rs	62.50%	6 Missing ⚠️
proxy/src/upstream/connection_counting.rs	0.00%	5 Missing ⚠️
async-std/src/server/unix.rs	0.00%	3 Missing ⚠️
server-common/src/config.rs	62.50%	3 Missing ⚠️
server-common/src/server_handle.rs	40.00%	3 Missing ⚠️
tokio/src/server/unix.rs	0.00%	3 Missing ⚠️
websockets/src/websocket_connection.rs	50.00%	2 Missing ⚠️
http/src/upgrade.rs	75.00%	1 Missing ⚠️
sse/src/lib.rs	0.00%	1 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##            0.3.x     #623      +/-   ##
==========================================
- Coverage   50.10%   49.94%   -0.16%     
==========================================
  Files         184      183       -1     
  Lines        7153     6967     -186     
==========================================
- Hits         3584     3480     -104     
+ Misses       3569     3487      -82     

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