pubsub/awssnssqs: Use PublishBatch for sending messages in SNS

[vangent]

Fixes #3285.

[codecov]

Codecov Report

Merging #3288 (6a4e843) into master (7b6de3c) will increase coverage by 0.02%.
Report is 1 commits behind head on master.
The diff coverage is 88.18%.

❗ Current head 6a4e843 differs from pull request most recent head a9e9a6d. Consider uploading reports for the commit a9e9a6d to get more accurate results

@@            Coverage Diff             @@
##           master    #3288      +/-   ##
==========================================
+ Coverage   77.41%   77.43%   +0.02%     
==========================================
  Files         104      104              
  Lines       13855    13908      +53     
==========================================
+ Hits        10726    10770      +44     
- Misses       2373     2378       +5     
- Partials      756      760       +4     

Files Changed	Coverage Δ
pubsub/awssnssqs/awssnssqs.go	84.75% <88.07%> (+0.64%)	⬆️
blob/fileblob/fileblob.go	82.09% <100.00%> (ø)

... and 2 files with indirect coverage changes