initial design/api for progressive ramp up

[ldemailly]

(will) Fix #717

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.1 ⚠️

Comparison is base (fecd9f3) 89.7% compared to head (2a443db) 89.7%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #718     +/-   ##
========================================
- Coverage    89.7%   89.7%   -0.1%     
========================================
  Files          16      16             
  Lines        4001    4001             
========================================
- Hits         3590    3587      -3     
- Misses        262     263      +1     
- Partials      149     151      +2     

Impacted Files	Coverage Δ
periodic/periodic.go	93.7% <ø> (ø)

... and 1 file with indirect coverage changes

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