Upgrade golang-migrate/migrate to v4 (#1384)

[adnxn]

What this PR does:
changes mattes/migrate path to golang-migrate/migrate, upgrades to v4.7.0, and corresponding changes to migrate api use in pkg/configs/db/postgres/postgres.go

ran go mod vendor and go mod tidy

Which issue(s) this PR fixes:
Fixes #1384

[jtlisi]

I have one minor nit, otherwise this looks good to me. Have you tested it locally?

[jtlisi]

Should this be a warn level log? It looks like it should be a debug level log since it is expected behavior?

[jtlisi]

LGTM

[bboreham]

Crashes immediately on startup for me, with -target configs:

no scheme
database migrations initialization failed
github.com/cortexproject/cortex/pkg/configs/db/postgres.New
	/go/src/github.com/cortexproject/cortex/pkg/configs/db/postgres/postgres.go:80
github.com/cortexproject/cortex/pkg/configs/db.New
	/go/src/github.com/cortexproject/cortex/pkg/configs/db/db.go:88
github.com/cortexproject/cortex/pkg/cortex.(*Cortex).initConfigs
	/go/src/github.com/cortexproject/cortex/pkg/cortex/modules.go:378
github.com/cortexproject/cortex/pkg/cortex.(*Cortex).initModule
	/go/src/github.com/cortexproject/cortex/pkg/cortex/cortex.go:232
github.com/cortexproject/cortex/pkg/cortex.(*Cortex).init
	/go/src/github.com/cortexproject/cortex/pkg/cortex/cortex.go:226
github.com/cortexproject/cortex/pkg/cortex.New
	/go/src/github.com/cortexproject/cortex/pkg/cortex/cortex.go:179
main.main
	/go/src/github.com/cortexproject/cortex/cmd/cortex/main.go:73
runtime.main
	/usr/local/go/src/runtime/proc.go:203
runtime.goexit
	/usr/local/go/src/runtime/asm_amd64.s:1357
error initialising module: configs
github.com/cortexproject/cortex/pkg/cortex.(*Cortex).initModule
	/go/src/github.com/cortexproject/cortex/pkg/cortex/cortex.go:233
github.com/cortexproject/cortex/pkg/cortex.(*Cortex).init
	/go/src/github.com/cortexproject/cortex/pkg/cortex/cortex.go:226
github.com/cortexproject/cortex/pkg/cortex.New
	/go/src/github.com/cortexproject/cortex/pkg/cortex/cortex.go:179
main.main
	/go/src/github.com/cortexproject/cortex/cmd/cortex/main.go:73
runtime.main
	/usr/local/go/src/runtime/proc.go:203
runtime.goexit
	/usr/local/go/src/runtime/asm_amd64.s:1357

[bboreham]

I got another crash today which could be related: #1972