Fix/concurrent map interaction

[alanprot]

What this PR does:

Fix race condition on /series.

ForReplicationSet can still invoke the callback after the quorum has reached and this can cause the metrics map to be write and read at the same time.

=== RUN   TestDistributor_MetricsForLabelMatchers_SingleSlowIngester
fatal error: concurrent map iteration and map write

goroutine 89 [running]:
runtime.throw({0x1efcf2d, 0x0})
	/usr/local/go/src/runtime/panic.go:1198 +0x71 fp=0xc000a47c28 sp=0xc000a47bf8 pc=0x10383b1
runtime.mapiternext(0xc000a47da0)
	/usr/local/go/src/runtime/map.go:858 +0x4eb fp=0xc000a47c98 sp=0xc000a47c28 pc=0x10112cb
github.com/cortexproject/cortex/pkg/distributor.(*Distributor).MetricsForLabelMatchers(0x0, {0x21c53d8, 0xc0007f13e0}, 0x1ec9e30, 0x4, {0xc000a47ee0, 0x1, 0x1})
	/Users/approtas/workspaces/cortex/pkg/distributor/distributor.go:985 +0x325 fp=0xc000a47e10 sp=0xc000a47c98 pc=0x1bef3e5
github.com/cortexproject/cortex/pkg/distributor.TestDistributor_MetricsForLabelMatchers_SingleSlowIngester(0x1)
	/Users/approtas/workspaces/cortex/pkg/distributor/distributor_test.go:1870 +0x30e fp=0xc000a47f70 sp=0xc000a47e10 pc=0x1c09b8e
testing.tRunner(0xc0001d0d00, 0x1f3d210)
	/usr/local/go/src/testing/testing.go:1259 +0x102 fp=0xc000a47fc0 sp=0xc000a47f70 pc=0x11156c2

Checklist

• Tests updated
• [ NA] Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]