allow further subscription even after client starts, to align with Java SDK's behavior

[beiwei30]

What is the purpose of the change

allow further subscription even after client starts, to align with Java SDK's behavior

Brief changelog

Verifying this change

XXXX

Follow this checklist to help us incorporate your contribution quickly and easily. Notice, it would be helpful if you could finish the following 5 checklist(the last one is not necessary)before request the community to review your PR.

• Make sure there is a Github issue filed for the change (usually before you start working on it). Trivial changes like typos do not require a Github issue. Your pull request should address just this issue, without pulling in other changes - one PR resolves one issue.
• Format the pull request title like [ISSUE #123] Fix UnknownException when host config not exist. Each commit in the pull request should have a meaningful subject line and body.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Write necessary unit-test(over 80% coverage) to verify your logic correction, more mock a little better when a cross-module dependency exists.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

[codecov-io]

Codecov Report

Merging #560 (f554daf) into master (e1d9be8) will increase coverage by 0.01%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #560      +/-   ##
==========================================
+ Coverage   24.21%   24.22%   +0.01%     
==========================================
  Files          48       48              
  Lines        5010     5011       +1     
==========================================
+ Hits         1213     1214       +1     
  Misses       3598     3598              
  Partials      199      199              

Impacted Files	Coverage Δ
consumer/push_consumer.go	11.46% <33.33%> (+0.12%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e1d9be8...f554daf. Read the comment docs.

[xujianhai666]

lgtm

[beiwei30]

thanks :)