refactor(repository): major server code refactoring

[jkowalski]

This removes big shared lock held for for the duration of each request
and replaces it with trivially short lock to capture the current
state of the server/repository before passing it to handlers.

Handlers are now limited to only accessing a small subset of Server
functionality to be able to better reason about them.

[codecov]

Codecov Report

Merging #1837 (63210b4) into master (9054fd0) will increase coverage by 0.00%.
The diff coverage is 72.65%.

@@           Coverage Diff           @@
##           master    #1837   +/-   ##
=======================================
  Coverage   72.73%   72.74%           
=======================================
  Files         394      396    +2     
  Lines       30829    30921   +92     
=======================================
+ Hits        22423    22492   +69     
- Misses       6722     6736   +14     
- Partials     1684     1693    +9     

Impacted Files	Coverage Δ
internal/server/api_restore.go	0.00% <0.00%> (ø)
internal/server/api_user.go	0.00% <0.00%> (ø)
snapshot/stats.go	100.00% <ø> (ø)
internal/server/api_mount.go	9.83% <4.00%> (+1.97%)	⬆️
internal/server/server_mount_manager.go	19.51% <19.51%> (ø)
internal/server/api_repo.go	38.32% <49.23%> (-1.24%)	⬇️
internal/server/api_tasks.go	53.57% <58.33%> (ø)
internal/server/api_policies.go	66.34% <60.00%> (ø)
internal/server/api_ui_pref.go	50.00% <62.50%> (ø)
internal/server/api_object_get.go	62.50% <66.66%> (ø)
... and 28 more

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