Simplify Distributor.push (#4240)
Simplify Distributor.push (#4240)

Instead of creating another goroutine to wait for the background
`sendSamples()` to finish, give the channels it is sending on a buffer
so that they can send without a listener.

Signed-off-by: Bryan Boreham <de652539b24efdf404b95def4635bf3dbfd846aa@gmail.com>
grafana/loki,grafana/loki,grafana/loki,grafana/loki,grafana/loki,grafana/loki