Don't block lock in forSeriesMatching callback

[codesome]

stream.Send here

cortex/pkg/ingester/ingester.go

Lines 411 to 413 in 05b35bb

	err = stream.Send(&client.QueryStreamResponse{
	Timeseries: batch,
	})

is time consuming, in-turn blocking the lock in forSeriesMatching.

This PR breaks the callbacks into 2 callbacks, where second callback is called in intervals outside the critical sections of the lock.

• Unit tests

/cc @tomwilkie

[tomwilkie]

Looks good @codesome! Left some feedback. I'm actually going to deploy and test this, so might do the feedback myself if thats okay.

[codesome]

@tomwilkie I'll be unavailable for ~35h from now (got a long flight ahead), so please go ahead!

[codesome]

PS: As this function was used elsewhere too where there was no notion of batch, send etc, I tried to keep the names more general.