Clamp scheduling delay to 6 months

[dervoeti]

Motivation

Fixes #1772 (long durations cause panics in DelayQueue.html#method.insert_at and DelayQueue.html#method.reset_at).

Solution

Clamp the scheduling delay to a maximum of 6 months. This value is now also used as fallback if checked_add fails.

[clux]

clean solution. thank you very much!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.5%. Comparing base (acb5625) to head (dd00553).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1779     +/-   ##
=======================================
+ Coverage   76.5%   76.5%   +0.1%     
=======================================
  Files         84      84             
  Lines       7895    7896      +1     
=======================================
+ Hits        6034    6038      +4     
+ Misses      1861    1858      -3     

Files with missing lines	Coverage Δ
kube-runtime/src/controller/mod.rs	30.2% <ø> (ø)
kube-runtime/src/scheduler.rs	96.1% <100.0%> (+1.4%)	⬆️

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[nightkr]

Implementation LGTM. Just a few comment nits.