support sticky sessions for load balancer

[samanhappy]

resolves #795

Code changes:

• add sticky configs in BaseLoadBalancer
• choose server using BaseLoadBalancer first, if nil use specific load balancer
• introduce consistent hash

[codecov-commenter]

Codecov Report

Base: 79.25% // Head: 76.83% // Decreases project coverage by -2.42% ⚠️

Coverage data is based on head (e71a2a8) compared to base (b67c5ff).
Patch coverage: 79.74% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #796      +/-   ##
==========================================
- Coverage   79.25%   76.83%   -2.43%     
==========================================
  Files         101      103       +2     
  Lines       11188    11577     +389     
==========================================
+ Hits         8867     8895      +28     
- Misses       1826     2174     +348     
- Partials      495      508      +13     

Impacted Files	Coverage Δ
pkg/filters/proxy/loadbalance.go	86.61% <79.22%> (-12.25%)	⬇️
pkg/filters/proxy/server.go	100.00% <100.00%> (ø)
pkg/util/jmxtool/agent_controller.go	46.66% <0.00%> (-15.10%)	⬇️
pkg/cluster/op.go	64.63% <0.00%> (-7.56%)	⬇️
pkg/cluster/syncer.go	77.56% <0.00%> (-5.77%)	⬇️
pkg/filters/requestadaptor/requestadaptor.go	75.46% <0.00%> (-1.32%)	⬇️
pkg/cluster/member.go	70.81% <0.00%> (-1.08%)	⬇️
pkg/object/mqttproxy/broker.go	74.79% <0.00%> (-1.01%)	⬇️
pkg/filters/responseadaptor/responseadaptor.go	68.69% <0.00%> (-0.68%)	⬇️
pkg/cluster/cluster.go	59.57% <0.00%> (-0.53%)	⬇️
... and 6 more

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