Update ai3/go-common

go.mod

go.sum

...or/git.autistici.org/ai3/go-common/clientutil/balancer.go

vendor/git.autistici.org/ai3/go-common/go.mod

vendor/git.autistici.org/ai3/go-common/go.sum

vendor/github.com/cenkalti/backoff/v4/.gitignore

vendor/github.com/cenkalti/backoff/v4/.travis.yml

vendor/github.com/cenkalti/backoff/v4/LICENSE

vendor/github.com/cenkalti/backoff/v4/README.md

vendor/github.com/cenkalti/backoff/v4/backoff.go

vendor/github.com/cenkalti/backoff/v4/context.go

vendor/github.com/cenkalti/backoff/v4/exponential.go

vendor/github.com/cenkalti/backoff/v4/go.mod

vendor/github.com/cenkalti/backoff/v4/retry.go

vendor/github.com/cenkalti/backoff/v4/ticker.go

vendor/github.com/cenkalti/backoff/v4/timer.go

vendor/github.com/cenkalti/backoff/v4/tries.go

vendor/modules.txt