*: fix the bug that the condition after show variables does not take …

[zhyass]

…effect #730

[summary]

1. Remove force_eof in some statement.
2. Completing show statement syntax.

[test case] src/vendor/github.com/xelabs/go-mysqlstack/sqlparser/show_test.go

[patch codecov] src/vendor/github.com/xelabs/go-mysqlstack/sqlparser/ast.go 96.5% src/vendor/github.com/xelabs/go-mysqlstack/sqlparser/sql.go 92.0%

[codecov[bot]]

Codecov Report

Merging #731 (86c0b24) into master (e71eeff) will decrease coverage by 0%. The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #731   +/-   ##
=====================================
- Coverage      89%    89%   -1%     
=====================================
  Files         133    133           
  Lines        9478   9478           
=====================================
- Hits         8475   8473    -2     
- Misses        665    666    +1     
- Partials      338    339    +1     

Impacted Files	Coverage Δ
src/proxy/query.go	93% <ø> (ø)
src/backend/txn.go	87% <0%> (-1%)	:arrow_down:
src/executor/engine/merge_join.go	96% <0%> (ø)

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[BohuTANG]

LGTM