Distributor registers logproto.Pusher service to receive logs via GRPC

[codewithcheese]

What this PR does / why we need it:
Distributor registers the logproto.Pusher service so that logs can be pushed to the distributor using GRPC. GRPC has a number of benefits over REST and this change would allow clients to push to the distributor using GRPC instead of REST.

Special notes for your reviewer:
See discussion: https://app.slack.com/client/T05675Y01/CEPJRLQNL/thread/CEPJRLQNL-1605100868.117200

Checklist

• Documentation added
• Tests updated

Resolves #2921

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #2922 (44f6500) into master (f0d4adc) will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2922      +/-   ##
==========================================
- Coverage   61.72%   61.67%   -0.06%     
==========================================
  Files         181      181              
  Lines       14724    14725       +1     
==========================================
- Hits         9089     9082       -7     
- Misses       4801     4812      +11     
+ Partials      834      831       -3     

Impacted Files	Coverage Δ
pkg/loki/modules.go	3.47% <0.00%> (-0.02%)	⬇️
pkg/promtail/positions/positions.go	46.80% <0.00%> (-11.71%)	⬇️
pkg/logql/evaluator.go	91.64% <0.00%> (+0.40%)	⬆️
pkg/querier/queryrange/downstreamer.go	97.64% <0.00%> (+2.35%)	⬆️

[cyriltovena]

LGTM