refactor: client in msggateway

[rfyiamcool]

summary

1. Refactor the client.go code.
2. A sync.pool object pool has been added for high-frequency req structs. Although the current code uses the value object of req, subsequent operations such as decode and replyMessage cause the object to escape to the heap. This means that during compilation, the object has already been determined to be allocated on the heap. Therefore, since this object is a heap object, pointers can be used directly afterwards, as stack allocation and copying also have their costs. Furthermore, it's only when the req does not escape that it's allocated on the stack.

Since it's a heap object, using sync.pool for object pooling can reduce the overhead of garbage collection.

[sweep-ai]

Apply Sweep Rules to your PR?

• Apply: All new business logic should have corresponding unit tests.
• Apply: Refactor large functions to be more modular.

[codecov]

Codecov Report

Merging #1343 (9bd59dc) into main (3d3781f) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main   #1343   +/-   ##
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  Coverage   2.41%   2.41%           
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  Files         13      13           
  Lines        786     786           
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  Hits          19      19           
  Misses       764     764           
  Partials       3       3