[ISSUE #611] Fix restart the same push consumer failed

[maixiaohai]

What is the purpose of the change

issue #611

Brief changelog

1. add func to remove useless client from clientMap
2. remove client instance when shutdown called

Verifying this change

XXXX

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[codecov-io]

Codecov Report

Merging #612 (dfe5998) into master (6cd3181) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #612      +/-   ##
==========================================
- Coverage   24.46%   24.46%   -0.01%     
==========================================
  Files          48       48              
  Lines        5047     5049       +2     
==========================================
  Hits         1235     1235              
- Misses       3609     3611       +2     
  Partials      203      203              

Impacted Files	Coverage Δ
internal/client.go	0.00% <0.00%> (ø)

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