Wrap progress updates in a mutex

[jonjohnsonjr]

Atomically incrementing the number of bytes written isn't sufficient if
we're sending the updates out of order.

I ran the progress tests 50 times after this change and they passed.

[codecov-commenter]

Codecov Report

Merging #1402 (9dc0cf4) into main (59b5c06) will increase coverage by 0.01%.
The diff coverage is 96.92%.

❗ Current head 9dc0cf4 differs from pull request most recent head b3f8daf. Consider uploading reports for the commit b3f8daf to get more accurate results

@@            Coverage Diff             @@
##             main    #1402      +/-   ##
==========================================
+ Coverage   74.14%   74.15%   +0.01%     
==========================================
  Files         113      114       +1     
  Lines        8450     8455       +5     
==========================================
+ Hits         6265     6270       +5     
  Misses       1579     1579              
  Partials      606      606              

Impacted Files	Coverage Δ
pkg/v1/remote/write.go	63.50% <93.54%> (-1.11%)	⬇️
pkg/v1/remote/multi_write.go	62.92% <100.00%> (ø)
pkg/v1/remote/progress.go	100.00% <100.00%> (ø)

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