minor fixes and improvements for boltdb shipper

[sandeepsukhani]

What this PR does / why we need it:
It includes the following fixes and improvements:

1. Continue uploading of other tables during failures otherwise a problem in single table's upload would avoid other tables from getting uploaded.
2. Remove .temp files while loading dbs during startup. A .temp file was left off due to ingester getting OOM killed while uploading a table which caused problems in uploading that table when ingester restarted.
3. Remove empty table folders while loading tables during startup. This is just to clean up unwanted folders.

Checklist

• Tests updated

[codecov-commenter]

Codecov Report

Merging #2530 into master will decrease coverage by 0.09%.
The diff coverage is 43.75%.

@@            Coverage Diff             @@
##           master    #2530      +/-   ##
==========================================
- Coverage   63.68%   63.58%   -0.10%     
==========================================
  Files         163      163              
  Lines       14305    14306       +1     
==========================================
- Hits         9110     9097      -13     
- Misses       4481     4499      +18     
+ Partials      714      710       -4     

Impacted Files	Coverage Δ
...kg/storage/stores/shipper/uploads/table_manager.go	68.10% <36.36%> (+0.03%)	⬆️
pkg/storage/stores/shipper/uploads/table.go	68.15% <60.00%> (-0.48%)	⬇️
pkg/promtail/positions/positions.go	46.49% <0.00%> (-13.16%)	⬇️
pkg/logql/evaluator.go	92.47% <0.00%> (-0.41%)	⬇️
pkg/promtail/targets/file/tailer.go	78.65% <0.00%> (+4.49%)	⬆️

[owen-d]

lgtm