Fix potential data race in Column.fieldPath

[yonekawa]

xorm's core has potential data race here

For example, my test code(simplified) with parallel.

var db *xorm.Engine
func TestMain(m *testing.M) {
    db, _ = xorm.NewEngine("mysql", "...")
}

func TestA(t *testing.T) {
	t.Parallel()

	_, err := db.Where("name = ?", "A").Delete(Struct{})
	if err != nil {
		t.Error(err)
		t.FailNow()
	}
}

func TestB(t *testing.T) {
	t.Parallel()

	_, err := db.Where("name = ?", "B").Delete(Struct{})
	if err != nil {
		t.Error(err)
		t.FailNow()
	}
}

When I run go test with -race option, detect DATA RACE.

WARNING: DATA RACE
Read at 0x00c4201583e0 by goroutine 7:
  github.com/yonekawa/example/vendor/github.com/go-xorm/core.(*Column).ValueOfV()
      /Users/yonekawa/project/vendor/github.com/go-xorm/core/column.go:138 +0x250
  github.com/yonekawa/example/vendor/github.com/go-xorm/core.(*Column).ValueOf()
      /Users/yonekawa/project/vendor/github.com/go-xorm/core/column.go:119 +0xa1
  github.com/yonekawa/example/vendor/github.com/go-xorm/xorm.buildConds()
      /Users/yonekawa/project/vendor/github.com/go-xorm/xorm/statement.go:518 +0x374
  github.com/yonekawa/example/vendor/github.com/go-xorm/xorm.(*Statement).buildConds()
      /Users/yonekawa/project/vendor/github.com/go-xorm/xorm/statement.go:1104 +0x207
  github.com/yonekawa/example/vendor/github.com/go-xorm/xorm.(*Statement).genConds()
      /Users/yonekawa/project/vendor/github.com/go-xorm/xorm/statement.go:1110 +0x20c
  github.com/yonekawa/example/vendor/github.com/go-xorm/xorm.(*Session).Delete()
      /Users/yonekawa/project/vendor/github.com/go-xorm/xorm/session_delete.go:100 +0x314
  github.com/yonekawa/example/core/infra/database/a.TestB()
      /Users/yonekawa/project/core/infra/database/a/connection_test.go:23 +0x1f4
  testing.tRunner()
      /usr/local/Cellar/go/1.7.4_1/libexec/src/testing/testing.go:610 +0xc9

Previous write at 0x00c4201583e0 by goroutine 6:
  strings.genSplit()
      /usr/local/Cellar/go/1.7.4_1/libexec/src/strings/strings.go:259 +0x390
  strings.Split()
      /usr/local/Cellar/go/1.7.4_1/libexec/src/strings/strings.go:287 +0x72
  github.com/yonekawa/example/vendor/github.com/go-xorm/core.(*Column).ValueOfV()
      /Users/yonekawa/project/vendor/github.com/go-xorm/core/column.go:125 +0xe67
  github.com/yonekawa/example/vendor/github.com/go-xorm/core.(*Column).ValueOf()
      /Users/yonekawa/project/vendor/github.com/go-xorm/core/column.go:119 +0xa1
  github.com/yonekawa/example/vendor/github.com/go-xorm/xorm.buildConds()
      /Users/yonekawa/project/vendor/github.com/go-xorm/xorm/statement.go:518 +0x374
  github.com/yonekawa/example/vendor/github.com/go-xorm/xorm.(*Statement).buildConds()
      /Users/yonekawa/project/vendor/github.com/go-xorm/xorm/statement.go:1104 +0x207
  github.com/yonekawa/example/vendor/github.com/go-xorm/xorm.(*Statement).genConds()
      /Users/yonekawa/project/vendor/github.com/go-xorm/xorm/statement.go:1110 +0x20c
  github.com/yonekawa/example/vendor/github.com/go-xorm/xorm.(*Session).Delete()
      /Users/yonekawa/project/vendor/github.com/go-xorm/xorm/session_delete.go:100 +0x314
  github.com/yonekawa/example/core/infra/database/a.TestA()
      /Users/yonekawa/project/core/infra/database/a/connection_test.go:13 +0x1f4
  testing.tRunner()
      /usr/local/Cellar/go/1.7.4_1/libexec/src/testing/testing.go:610 +0xc9

I think we don't need to cache fieldPath into Column.
Because, same logic is exists here without cache result.

[lunny]

Thanks!