support offset

[mxlxm]

fix #847,#739,#647.

[LeoQuote]

感谢 pr, 请先rebase 一下到最新分支.

[LeoQuote]

测试没过, 不兼容原有的 limit 关键词改写

[codecov]

Codecov Report

Merging #1172 (bf88b63) into master (f24d059) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head bf88b63 differs from pull request most recent head 9e66c17. Consider uploading reports for the commit 9e66c17 to get more accurate results

@@            Coverage Diff             @@
##           master    #1172      +/-   ##
==========================================
+ Coverage   77.89%   77.92%   +0.02%     
==========================================
  Files          76       76              
  Lines       11941    11948       +7     
==========================================
+ Hits         9302     9311       +9     
+ Misses       2639     2637       -2     

Impacted Files	Coverage Δ
sql/engines/mysql.py	92.27% <100.00%> (+0.23%)	⬆️
sql/engines/tests.py	99.82% <100.00%> (+0.17%)	⬆️

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