*: add config lower-case-table-names to support case insensitive tabl…

[zhyass]

…e name #725

[summary]

1. lower-case-table-names: If set 0, table names are stored as specified and comparisons are case-sensitive. If set 1, table names are stored in lowercase on disk and comparisons are not case-sensitive.
2. Change the type of show.Database to TableIdent to support case insensitive.

[test case] src/ctl/v1/radon_test.go src/proxy/initdb_test.go src/proxy/proxy_test.go src/proxy/query_test.go src/vendor/github.com/xelabs/go-mysqlstack/sqlparser/ast_test.go

[patch codecov] src/ctl/v1/radon.go 92.8% src/proxy/initdb.go 94.4% src/proxy/proxy.go 86.5% src/proxy/query.go 94.8% src/vendor/github.com/xelabs/go-mysqlstack/sqlparser/ast_funcs.go 94.7% src/vendor/github.com/xelabs/go-mysqlstack/sqlparser/sql.go 92.0%

[codecov[bot]]

Codecov Report

Merging #726 (1bdd2f4) into master (23eda68) will decrease coverage by 0%. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #726   +/-   ##
=====================================
- Coverage      89%    89%   -1%     
=====================================
  Files         133    133           
  Lines        9464   9478   +14     
=====================================
+ Hits         8459   8471   +12     
- Misses        666    667    +1     
- Partials      339    340    +1     

Impacted Files	Coverage Δ
src/ctl/v1/radon.go	90% <100%> (+<1%)	:arrow_up:
src/proxy/initdb.go	89% <100%> (+1%)	:arrow_up:
src/proxy/proxy.go	81% <100%> (+<1%)	:arrow_up:
src/proxy/query.go	93% <100%> (+<1%)	:arrow_up:
src/proxy/show.go	82% <100%> (ø)
src/proxy/spanner.go	92% <100%> (+<1%)	:arrow_up:
src/executor/engine/merge_join.go	95% <0%> (-2%)	:arrow_down:

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[hustjieke]

LGTM.