Remove support for Go1.6-1.8

[dfawley]

This change is 

[menghanl]

Missing README changes.

Reviewed 51 of 51 files at r1.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @dfawley)

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channelz/service/service_test.go, line 54 at r1 (raw file):

// This behavior will make reflect.DeepEqual fail upon unset time.Time field,
// and cause false positive fatal error.
// TODO: Go1.7 is no longer supported - does this need a change?

@lyuxuan to take a look.

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credentials/credentials.go, line 315 at r1 (raw file):

func init() {
	cipherSuiteLookup[tls.TLS_RSA_WITH_AES_128_CBC_SHA256] = "TLS_RSA_WITH_AES_128_CBC_SHA256"

Merge these with the map above.

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credentials/credentials.go, line 327 at r1 (raw file):

//
// If cfg is nil, a new zero tls.Config is returned.
func cloneTLSConfig(cfg *tls.Config) *tls.Config {

I think we can just inline this function. Add a TODO?

[dfawley]

Done

Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @menghanl)

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channelz/service/service_test.go, line 54 at r1 (raw file):

Previously, menghanl (Menghan Li) wrote…

@lyuxuan to take a look.

Let's follow-up on that in a separate PR.

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credentials/credentials.go, line 315 at r1 (raw file):

Previously, menghanl (Menghan Li) wrote…

Merge these with the map above.

Done.

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credentials/credentials.go, line 327 at r1 (raw file):

Previously, menghanl (Menghan Li) wrote…

I think we can just inline this function. Add a TODO?

Done.

[menghanl]

Reviewed 2 of 2 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved

[menghanl]

And change labels to keep this in the release notes.

[mitar]

It would be great if you would version your install links. Now go get -u google.golang.org/grpc fails on go 1.8 in my Docker file while before it was working. Maybe consider providing things like go get -u google.golang.org/grpc-v1.16 to be able to pin to a specific version.

[dfawley]

@mitar I don't think go get works like that. What you're describing is more like how modules work (experimental in go1.11). Otherwise, you will need to vendor your dependencies (manually or by using a tool like dep) if you want to use an unsupported version of Go.