Boltdb shipper query readiness

[sandeepsukhani]

What this PR does / why we need it:

1. Queriers would open and reuse the existing index file in cache dir during startup.
2. Adds support for being query ready for configured last n days by keeping the index always downloaded for that duration.

Special notes for your reviewer:

1. Query readiness only considers daily tables for simplicity, we already have made it mandatory to have daily tables for boltdb-shipper, long back.
2. I have removed the code for checking the mtime of already downloaded files during sync operation because we never upload a file with the same name but with different content. This is done to simplify the code for Sync operation.

Checklist

• Documentation added
• Tests updated

[codecov-io]

Codecov Report

Merging #2911 (4cfe998) into master (1c8dd3e) will increase coverage by 0.03%.
The diff coverage is 57.93%.

@@            Coverage Diff             @@
##           master    #2911      +/-   ##
==========================================
+ Coverage   61.77%   61.81%   +0.03%     
==========================================
  Files         182      182              
  Lines       14772    14863      +91     
==========================================
+ Hits         9126     9187      +61     
- Misses       4811     4830      +19     
- Partials      835      846      +11     

Impacted Files	Coverage Δ
pkg/storage/stores/shipper/shipper_index_client.go	0.00% <0.00%> (ø)
.../storage/stores/shipper/downloads/table_manager.go	56.77% <50.00%> (-3.67%)	⬇️
pkg/storage/stores/shipper/downloads/table.go	65.83% <72.72%> (+0.71%)	⬆️
pkg/querier/queryrange/downstreamer.go	95.29% <0.00%> (-2.36%)	⬇️
pkg/canary/comparator/comparator.go	76.36% <0.00%> (-1.82%)	⬇️
pkg/logql/evaluator.go	91.64% <0.00%> (+0.40%)	⬆️
pkg/promtail/targets/file/filetarget.go	66.19% <0.00%> (+2.11%)	⬆️
pkg/promtail/positions/positions.go	58.51% <0.00%> (+11.70%)	⬆️

[cyriltovena]

LGTM.

I think you just need to tm.Stop line 70 in the table_manager.go