fix: upstream nodes filed IP occupation.

[AlinsRan]

Type of change:

• Bugfix
• New feature provided
• Improve performance
• Backport patches

What this PR does / why we need it:

#674

Development scheme:

• Implements an UpstreamService to maintain the servicename->upstreamname relationship, which exists in the cache.
• Call link: Watch Resource -> Controller Handler -> Translator -> Upstream -> UpstreamService -> cache -> apisix
• Upstream (create delete update) -> UpstreamService(create delete update)
• Endpoint (types.EvnetDelete) -> UpstreamService(delete)

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[codecov-commenter]

Codecov Report

Merging #1064 (f521432) into master (5756273) will decrease coverage by 0.16%.
The diff coverage is 32.31%.

@@            Coverage Diff             @@
##           master    #1064      +/-   ##
==========================================
- Coverage   32.08%   31.92%   -0.17%     
==========================================
  Files          78       79       +1     
  Lines        8795     8927     +132     
==========================================
+ Hits         2822     2850      +28     
- Misses       5666     5768     +102     
- Partials      307      309       +2     

Impacted Files	Coverage Δ
pkg/apisix/apisix.go	61.36% <ø> (ø)
pkg/ingress/endpoint.go	0.00% <0.00%> (ø)
pkg/apisix/upstreamservicerelation.go	3.70% <3.70%> (ø)
pkg/apisix/cluster.go	31.89% <33.33%> (+<0.01%)	⬆️
pkg/apisix/nonexistentclient.go	43.27% <61.22%> (-1.54%)	⬇️
pkg/apisix/cache/memdb.go	75.55% <76.47%> (+0.07%)	⬆️
pkg/apisix/upstream.go	39.73% <100.00%> (+2.49%)	⬆️

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