.*: support help statement #710

[hustjieke]

[summary] support "help statement" for users to get help online information from mysql.

[test case] src/proxy/help_test.go src/vendor/github.com/xelabs/go-mysqlstack/sqlparser/parse_test.go

[patch codecov] src/proxy/help.go 100% src/vendor/github.com/xelabs/go-mysqlstack/sqlparser/ast.go 96.6% src/vendor/github.com/xelabs/go-mysqlstack/sqlparser/rewriter.go 64.7%

[codecov[bot]]

Codecov Report

Merging #711 (bc4d9c0) into master (988f634) will decrease coverage by 0%. The diff coverage is 57%.

@@          Coverage Diff          @@
##           master   #711   +/-   ##
=====================================
- Coverage      89%    89%   -1%     
=====================================
  Files         130    131    +1     
  Lines        9264   9271    +7     
=====================================
  Hits         8252   8252           
- Misses        671    675    +4     
- Partials      341    344    +3     

Impacted Files	Coverage Δ
src/proxy/query.go	86% <50%> (-1%)	:arrow_down:
src/proxy/help.go	100% <100%> (ø)
src/router/api.go	88% <0%> (-3%)	:arrow_down:
src/executor/engine/merge_join.go	95% <0%> (-2%)	:arrow_down:

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[BohuTANG]

LGTM. The coverage is no releated to this patch.