improvements for boltdb-shipper compactor

[sandeepsukhani]

What this PR does / why we need it:
Does the following improvements to boltdb-shipper compactor:

1. Run the compaction process on startup instead of waiting for the first tick.
2. Make the interval at which compaction runs configurable and reduce the default interval to 2 hours because lesser the files faster the query response.
3. Run the compactor service when using loki with boltdb-shipper in a single binary mode without clustering.

[codecov-commenter]

Codecov Report

Merging #2640 into master will decrease coverage by 1.47%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2640      +/-   ##
==========================================
- Coverage   62.87%   61.40%   -1.48%     
==========================================
  Files         170      170              
  Lines       15051    13187    -1864     
==========================================
- Hits         9464     8097    -1367     
+ Misses       4827     4349     -478     
+ Partials      760      741      -19     

Impacted Files	Coverage Δ
pkg/loki/loki.go	0.00% <0.00%> (ø)
pkg/storage/stores/shipper/compactor/compactor.go	0.00% <0.00%> (ø)
pkg/logql/step_evaluator.go	57.14% <0.00%> (-9.53%)	⬇️
pkg/logql/marshal/labels.go	66.66% <0.00%> (-8.34%)	⬇️
pkg/logproto/timestamp.go	40.00% <0.00%> (-6.81%)	⬇️
pkg/logentry/stages/labeldrop.go	53.33% <0.00%> (-6.67%)	⬇️
pkg/chunkenc/encoding_helpers.go	59.25% <0.00%> (-6.37%)	⬇️
pkg/logql/marshal/tail.go	66.66% <0.00%> (-6.07%)	⬇️
pkg/promtail/api/types.go	14.28% <0.00%> (-5.72%)	⬇️
pkg/logql/vector.go	81.81% <0.00%> (-5.69%)	⬇️
... and 162 more

[slim-bean]

LGTM!

[oleksandrsemak]

Hey @sandeepsukhani @slim-bean is it right example of compactor? It has to be in config: ?
Image: grafana/loki:1.6.0
field compactor not found in type main.Config

[slim-bean]

The compactor is not available in a release yet, only in master builds. Hold tight though, a release is coming soon!