A lot of memory allocation optimizations

[atercattus]

Zero-allocation in hot paths for mysql/client.
!! Changed Resultset.Values public API !!

This package is very fond of temporarily memory allocation. I like this package, but its gc pressure isn't acceptable for me. So I did a number of small (I have tried) changes in code base for removing that allocations.
I don't sure it's acceptable for you, but I considered it my duty to send PR :)

Before changes:

After changes:

Typical usage:

r, err := conn.Execute(sql)
//... Working with r
r.Close() // New call for memory reusing

[siddontang]

Sorry for the late response @atercattus

I am very busy these days, I will review your PR ASAP.

[bobotu]

LGTM