Bump minimum Go version from 1.7 to 1.8.

[michael-berlin]

No description provided.

[o1egl]

I think we should wait for 1.8.x golang/go#19182

[alainjobart]

That issue will most likely not apply to us. It seems to be an issue where an Atomic value is incremented in a loop, and the compiler optimizes away some code it shouldn't necessarily take away. We don't really do these gratuitous loops anywhere in our code...

YouTube has been running with 1.8 for a while now, and we look at our variables (most instances of atomic counters are for monitoring), they're fine.

[sougou]

We come close here https://github.com/youtube/vitess/blob/master/go/pools/resource_pool.go#L172, but it's a CAS operation that also does other things, which will require the compiler to generate code :).