Add timeout to tcp connections

[Systemcluster]

Depending on system configuration, waiting for a connection error when checking for a running server can take a long time. In my local WSL installation, this is causing sccache to take several minutes to start up.

This PR replaces the two calls to connect with connect_timeout and adds a SCCACHE_CONNECTION_TIMEOUT environment variable for configuration. The code for reading the env var is copied from server.rs:

sccache/src/server.rs

Lines 94 to 101 in 9958e7c

	/// Get the time the server should idle for before shutting down, in seconds.
	fn get_idle_timeout() -> u64 {
	// A value of 0 disables idle shutdown entirely.
	env::var("SCCACHE_IDLE_TIMEOUT")
	.ok()
	.and_then(|s| s.parse().ok())
	.unwrap_or(DEFAULT_IDLE_TIMEOUT)
	}

[Systemcluster]

This could be much lower, but leaving it this high aligns it more closely with the current behavior. I'm running it with 2 locally.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 30.00000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 40.88%. Comparing base (0cc0c62) to head (7296b2b).
Report is 68 commits behind head on main.

Files	Patch %	Lines
src/client.rs	33.33%	2 Missing and 4 partials ⚠️
src/dist/client_auth.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2231      +/-   ##
==========================================
+ Coverage   30.91%   40.88%   +9.96%     
==========================================
  Files          53       55       +2     
  Lines       20112    20675     +563     
  Branches     9755     9799      +44     
==========================================
+ Hits         6217     8452    +2235     
- Misses       7922     8093     +171     
+ Partials     5973     4130    -1843     

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[Systemcluster]

@Xuanwo could you review this? I don't think it's a controversial change, but I can revert to calling connect without timeout if the environment variable isn't set otherwise.

[Xuanwo]

Thanks and sorry for the late, I only have some small questions.

[Xuanwo]

Overall, this idea looks good to me. However, I think it should be a config value in the [dist] section instead of an environment variable. Most distribution configurations aren't exposed to the environment for now.

The current name is a bit confusing from my point of view. I would expect something like tcp_connect_timeout.

[Systemcluster]

Should connect_to_server in src/client.rs and the functions that use it (connect_or_start_server in src/command.rs) take the config struct as an argument?