s3: Correct layout prefix, reduce connections

[fd0]

Closes #917

[codecov-io]

Codecov Report

Merging #918 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #918      +/-   ##
==========================================
+ Coverage   55.73%   55.78%   +0.04%     
==========================================
  Files         101      101              
  Lines        7907     7915       +8     
==========================================
+ Hits         4407     4415       +8     
  Misses       2860     2860              
  Partials      640      640

Impacted Files	Coverage Δ
src/restic/backend/s3/s3.go	79.44% <100%> (ø)	⬆️
src/restic/backend/layout_s3.go	91.3% <100%> (+4.63%)	⬆️

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