Improve: expand filterFuncCacheMap initial length

[alchemy-lee]

What this PR does:

1. pixiu initializes 7 filters in init.Run() function, but the initial value of filterFuncCacheMap is 4, it should be changed to 8.
2. the GenericServicePool has been initialized twice in NewDubboClient() and Init(), we can omit the second initialization.

Does this PR introduce a user-facing change?:

NO

[codecov-commenter]

Codecov Report

Merging #174 (2841428) into develop (1ee7018) will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #174      +/-   ##
===========================================
+ Coverage    42.28%   42.30%   +0.02%     
===========================================
  Files           33       33              
  Lines         1743     1742       -1     
===========================================
  Hits           737      737              
+ Misses         905      904       -1     
  Partials       101      101              

Impacted Files	Coverage Δ
pkg/client/dubbo/dubbo.go	21.15% <ø> (+0.20%)	⬆️

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