chore(router): periodic flush during pickup

[atzoum]

Description

Occasionally, router's pickup may take more than a few seconds, e.g. when it is going deep in its large backlog attempting to find jobs eligible for processing.
To avoid having long pauses and large bursts in workers, the pickup loop will now be flushing its jobs every 2 seconds.

Notion Ticket

Link

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[codecov]

Codecov Report

Patch coverage: 88.46% and project coverage change: +0.08 🎉

Comparison is base (5021989) 68.07% compared to head (d9f74c6) 68.15%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3497      +/-   ##
==========================================
+ Coverage   68.07%   68.15%   +0.08%     
==========================================
  Files         330      330              
  Lines       53577    53589      +12     
==========================================
+ Hits        36473    36525      +52     
+ Misses      14713    14674      -39     
+ Partials     2391     2390       -1     

Impacted Files	Coverage Δ
router/handle.go	74.50% <88.00%> (+1.29%)	⬆️
router/handle_lifecycle.go	94.40% <100.00%> (-0.98%)	⬇️

... and 10 files with indirect coverage changes

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