RFC: Add max concurrent hashes setting

[zeripath]

Some hash algorithms can be very costly to generate. This PR adds a setting to limit the maximum concurrent hashes performed at any one time.

[zeripath]

Related #14702

[zeripath]

conflicts resolved

[codecov-commenter]

Codecov Report

Merging #14713 (76d117f) into main (6021fbf) will decrease coverage by 0.03%.
The diff coverage is 30.57%.

@@            Coverage Diff             @@
##             main   #14713      +/-   ##
==========================================
- Coverage   44.07%   44.04%   -0.04%     
==========================================
  Files         682      682              
  Lines       82408    82457      +49     
==========================================
- Hits        36322    36317       -5     
- Misses      40176    40233      +57     
+ Partials     5910     5907       -3     

Impacted Files	Coverage Δ
cmd/admin.go	0.00% <0.00%> (ø)
modules/auth/sso/reverseproxy.go	8.51% <0.00%> (ø)
routers/events/events.go	0.00% <0.00%> (ø)
routers/install.go	0.00% <0.00%> (ø)
routers/private/manager.go	0.00% <0.00%> (ø)
routers/repo/blame.go	0.00% <0.00%> (ø)
routers/user/auth_openid.go	0.00% <0.00%> (ø)
services/archiver/archiver.go	72.78% <0.00%> (ø)
models/user.go	51.19% <21.42%> (-1.96%)	⬇️
routers/user/auth.go	12.30% <25.00%> (ø)
... and 18 more

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[6543]

do you have time to resolve the conflicts?

[zeripath]

conflicts resolved