utils: performance improvement for parseDateTime

[chanxuehong]

Description

I optimized the performance of parseDateTime in the PR #1098.

Now I find that parseDateTime can still be optimized. The implementation of this version is like parseBinaryDateTime, see

mysql/utils.go

Line 126 in 096feaa

func parseBinaryDateTime(num uint64, data []byte, loc *time.Location) (driver.Value, error) {

Before:
BenchmarkParseDateTime-8   	 5117752	       235 ns/op
Now:
BenchmarkParseDateTime-8   	16154996	        74.8 ns/op

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[chanxuehong]

I just saw #1113, please ignore this PR