Fix poolmanager terminates running function pod periodically

[life1347]

The pool manager keeps terminating function pod periodically even there are
traffic to the function. The root cause is that executor, poolmgr, newdeploy
manage their own functionServiceCache separately. And when router taps a
function, executor updates the access time of the function service entry in its
own cache without notifying executor types to do the update as well. Hence,
the access time of function service entry in poolmanager cache never gets updated.
Due to the access time never gets updated, the idle pod reaper in poolmanager
then thinks the function pod is in idle state and recycle it.

This PR removes the cache in executor itself, and when router tries to tap a function,
executor will call executor type to tap the function and update access time.

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This change is 

[codecov]

Codecov Report

Merging #1435 into master will not change coverage.
The diff coverage is 80%.

@@           Coverage Diff           @@
##           master    #1435   +/-   ##
=======================================
  Coverage   31.76%   31.76%           
=======================================
  Files          69       69           
  Lines        5562     5562           
=======================================
  Hits         1767     1767           
  Misses       3561     3561           
  Partials      234      234

Impacted Files	Coverage Δ
pkg/fission-cli/cmd/function/update.go	0% <0%> (ø)	⬆️
pkg/fission-cli/cmd/function/create.go	20.83% <88.88%> (ø)	⬆️

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