[INLONG-11703][SDK] Improve the cleanExpiredConnTicker select logic in the connpool of the Golang SDK

[gunli]

Fixes #11703

Motivation

Refactor

Modifications

1. rename connpool.recoverAndRebalance() to connpool.innerWork()
2. use a select/case chan to replace the default case.

Verifying this change

(Please pick either of the following options)

• This change is a trivial rework/code cleanup without any test coverage.

• This change is already covered by existing tests, such as:
  (please describe tests)

• This change added tests and can be verified as follows:

  (example:)

  • Added integration tests for end-to-end deployment with large payloads (10MB)
  • Extended integration test for recovery after broker failure

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)
• If a feature is not applicable for documentation, explain why?
• If a feature is not documented yet in this PR, please create a follow-up issue for adding the documentation