etcd rewrote its import path from coreos/etcd to go.etcd.io/etcd.
Changed the imports path in this commit, which also updates the code
version.

This lets us remove the github.com/ugorji/go/codec dependency, which
was pinned to a fairly old version. The net change is a loss of 30,000
lines of code in the vendor directory. (I first noticed this problem
because the outdated go/codec dependency was causing a dependency
failure when I tried to put Terraform and another project in the same
vendor directory.)

Note the version shows up funkily in go.mod, but I verified
visually it's the same commit as the "release-3.4" tag in
github.com/coreos/etcd. The etcd team plans to fix the release version
tagging in v3.5, which should be released soon.