Pool controller

[chungers]

The PR is the pool controller

• equivalent to the "Group" controller in managing a 'pool' of instances.
• uses FSM to implement throttle/ parallelism
• does not "overshoot" when provisioning when backends are too slow
• can easily scale down to 0 (Scale Group size down to 0 #717)

Once the pool controller is stable and examples put together, the Group controller will be deprecated in favor of a consistent schema and controller implementation using finite state machines than using instance counts and polling.

Signed-off-by: David Chung [email protected]

[codecov]

Codecov Report

Merging #896 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #896      +/-   ##
=========================================
- Coverage   48.56%   48.5%   -0.07%     
=========================================
  Files          84      84              
  Lines        7837    7837              
=========================================
- Hits         3806    3801       -5     
- Misses       3721    3725       +4     
- Partials      310     311       +1

Impacted Files	Coverage Δ
pkg/rpc/mux/server.go	42.7% <0%> (-5.21%)	⬇️

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