add built-in query object caching for reuse in the multiple queries with the same query string

[zhengchun]

htmlquery provides new methods QuerySelector and QuerySelectorAll, We can use compiled query object to reuse and avoid recompile the XPath query expression, this will shorter query speed time and improve application performance.

[subsr97]

@zhengchun
As I understand, you're suggesting to cache compiled XPath query expressions for use by FindOne and Find methods.

But wouldn't the overhead of maintaining a cache be greater than the overhead caused by recompiling XPath queries?

[zhengchun]

In fact, caching frequently used query objects can improve query performance. groupcache, I just caching Top N objects those the most frequent query object.

[zhengchun]

https://github.com/antchfx/htmlquery/blob/master/query_test.go#L47

goos: windows
goarch: amd64
pkg: github.com/antchfx/htmlquery
BenchmarkSelectorCache-4                20000000                55.2 ns/op
BenchmarkDisableSelectorCache-4           500000              3162 ns/op
PASS

[zhengchun]

# Add XPath selector object caching feature 14e3905