[ISSUE #151] Optimize PullConsumer API

[wenfengwang]

What is the purpose of the change

Optimize PullConsumer API

Brief changelog

refactor API of PullConsumer

Verifying this change

align apache/rocketmq-ons#8

[codecov-io]

Codecov Report

Merging #152 into native will decrease coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           native     #152      +/-   ##
==========================================
- Coverage   17.06%   16.94%   -0.12%     
==========================================
  Files          42       42              
  Lines        4402     4402              
==========================================
- Hits          751      746       -5     
- Misses       3555     3560       +5     
  Partials       96       96

Impacted Files	Coverage Δ
consumer/statistics.go	62.17% <0%> (-2.18%)	⬇️

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