Add /v3 suffix to go.mod and update import paths

[fiorix]

This is an attempt to resolve #11154 by completing the migration to Go
modules. This is a breaking change to anyone using Go older than 1.11.

On Go 1.11 or newer, with or without modules, the "go.etcd.io/etcd"
import path should still work.

Users that have already adopted modules are then encouraged to use the
new, versioned import path "go.etcd.io/etcd/v3" in their projects.

Please read https://github.com/etcd-io/etcd/blob/master/CONTRIBUTING.md#contribution-flow.

[fiorix]

Sending this mainly to see if travis would pass. No expectation that it will be merged.

[fiorix]

Interesting, the binaries such as etcd-dump-logs get stuck on the incompatible version:

can't load package: package go.etcd.io/etcd/tools/etcd-dump-logs: module go.etcd.io/etcd@latest (v3.3.15+incompatible) found, but does not contain package go.etcd.io/etcd/tools/etcd-dump-logs