Issue indexer queue redis support

[lunny]

This PR add a redis queue for issue indexer queue.

[codecov-io]

Codecov Report

Merging #6218 into master will decrease coverage by 0.08%.
The diff coverage is 10.09%.

@@            Coverage Diff             @@
##           master    #6218      +/-   ##
==========================================
- Coverage   40.41%   40.33%   -0.09%     
==========================================
  Files         404      405       +1     
  Lines       54140    54236      +96     
==========================================
- Hits        21883    21875       -8     
- Misses      29241    29345     +104     
  Partials     3016     3016

Impacted Files	Coverage Δ
modules/indexer/issues/queue_redis.go	0% <0%> (ø)
modules/setting/indexer.go	100% <100%> (ø)	⬆️
modules/indexer/issues/indexer.go	60.5% <36.84%> (-4.96%)	⬇️
modules/log/event.go	64.46% <0%> (-1.53%)	⬇️
models/repo_list.go	66.84% <0%> (-1.06%)	⬇️
routers/repo/view.go	41.08% <0%> (-1%)	⬇️

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