fix: Race when shutting down and opening WebSockets

[kylecarbs]

Adding to a WaitGroup while calling wait is a race condition. Surrounding
this in a mutex should solve the problem. Since context is used for
cancellation on all sockets, cleanup should occur properly.

See: https://github.com/coder/coder/runs/5701221057?check_suite_focus=true#step:10:98

[codecov]

Codecov Report

Merging #576 (f869e5e) into main (ff2f647) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #576      +/-   ##
==========================================
+ Coverage   63.20%   63.32%   +0.12%     
==========================================
  Files         109      195      +86     
  Lines       10132    11215    +1083     
  Branches        0       85      +85     
==========================================
+ Hits         6404     7102     +698     
- Misses       2998     3352     +354     
- Partials      730      761      +31     

Flag	Coverage Δ
unittest-go-	62.16% <100.00%> (+0.10%)	⬆️
unittest-go-macos-latest	57.92% <100.00%> (-0.19%)	⬇️
unittest-go-ubuntu-latest	61.12% <100.00%> (-0.26%)	⬇️
unittest-go-windows-2022	57.11% <100.00%> (?)
unittest-js	63.32% <ø> (?)

Impacted Files	Coverage Δ
coderd/coderd.go	96.42% <100.00%> (+0.10%)	⬆️
coderd/provisionerdaemons.go	59.15% <100.00%> (+0.64%)	⬆️
coderd/workspaceresources.go	58.37% <100.00%> (+0.76%)	⬆️
peer/conn.go	77.91% <0.00%> (-3.81%)	⬇️
cli/cliui/job.go	66.66% <0.00%> (-2.57%)	⬇️
peer/channel.go	83.04% <0.00%> (-2.34%)	⬇️
site/src/components/Icons/CoderIcon.tsx	100.00% <0.00%> (ø)
site/src/components/Workspace/constants.ts	100.00% <0.00%> (ø)
site/src/util/swr.ts	0.00% <0.00%> (ø)
site/src/components/Button/SplitButton.tsx	89.47% <0.00%> (ø)
... and 91 more

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