feat(loki): extended tailing (#764)

* refactor(querier/ingester): TailRequest Lookback window

Moves the specifications of the Lookback Window out of the
logproto.QueryRequest into it's own type logproto.Lookback.
This is required, because the Lookback Window will be used in the TailRequest
as well.

* feat(querier): parse Lookback from HTTP Request

* feat(logcli): send Lookback Window spec with tail request

* feat(querier): include historic entries in tail mode

Extends tailing by sending a configurable amount of historic entries with
before the live entries. This enables a behaviour that is closer to kubectl logs
-f and docker logs -f.

It is implemented by running a regular Query before subscribing to the ingesters.

* fix: adapt tests to Lookback change

* feat(querier): check all errors to make the linter happy

* fix(logproto): flatten Lookback window spec

Flattens the Lookback window spec into the individual queries

* fix(ingester): adapt test to Lookback flatten