Rft: Restructuring remoting metadata service

[justxuewei]

What this PR does:

Restructuring RemotingMetadataService to enhance the readability.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[codecov-commenter]

Codecov Report

Merging #1227 (8a6baae) into 3.0 (968650f) will decrease coverage by 3.68%.
The diff coverage is 50.60%.

@@            Coverage Diff             @@
##              3.0    #1227      +/-   ##
==========================================
- Coverage   59.53%   55.85%   -3.69%     
==========================================
  Files         259      272      +13     
  Lines       12737    12880     +143     
==========================================
- Hits         7583     7194     -389     
- Misses       4199     4792     +593     
+ Partials      955      894      -61     

Impacted Files	Coverage Δ
cluster/cluster_impl/available_cluster_invoker.go	66.66% <ø> (ø)
cluster/cluster_impl/broadcast_cluster_invoker.go	76.47% <ø> (ø)
cluster/cluster_impl/failfast_cluster_invoker.go	66.66% <ø> (ø)
cluster/cluster_impl/mock_cluster.go	0.00% <0.00%> (ø)
cluster/loadbalance/random.go	100.00% <ø> (ø)
...router/v3router/judger/list_string_match_judger.go	0.00% <0.00%> (ø)
...ster/router/v3router/judger/method_match_judger.go	0.00% <0.00%> (ø)
...er/router/v3router/judger/url_label_match_judge.go	0.00% <0.00%> (ø)
common/extension/auth.go	0.00% <ø> (ø)
common/extension/cluster.go	0.00% <ø> (ø)
... and 275 more

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