redis: fix deletion versioning

[calebdoxsey]

Summary

The redis implementation of the databroker is not currently incrementing the record version when a record is deleted. Because the version doesn't change, the record change isn't detected by sync and consumers will only pick up the change on restart.

Checklist

• reference any related issues
• updated docs
• updated unit tests
• updated UPGRADING.md
• add appropriate tag (improvement / bug / etc)
• ready for review

[codeclimate]

Code Climate has analyzed commit ae523e5 and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Merging #1871 (d468016) into master (a389139) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master   #1871     +/-   ##
========================================
- Coverage    59.5%   59.4%   -0.1%     
========================================
  Files         138     138             
  Lines        9643    9644      +1     
========================================
- Hits         5744    5735      -9     
- Misses       3302    3309      +7     
- Partials      597     600      +3     

Impacted Files	Coverage Δ
pkg/storage/redis/redis.go	74.2% <100.0%> (-1.0%)	⬇️
internal/controlplane/xdsmgr/xdsmgr.go	75.4% <0.0%> (-5.9%)	⬇️
internal/autocert/manager.go	65.5% <0.0%> (-1.4%)	⬇️