Do not keep old key versions on disk in Badger

[mschneider82]

Which problem is this PR solving?

Resolves #1916

Short description of the changes

set NumVersionsToKeep to 0 instead of default 1 acording to dgraph-io/badger#1228

[pavolloffay]

@mschneider82 could you please sign off the PR? (DCO check)

[codecov]

Codecov Report

Merging #2150 into master will increase coverage by 0.01%.
The diff coverage is 78.00%.

@@            Coverage Diff             @@
##           master    #2150      +/-   ##
==========================================
+ Coverage   96.14%   96.16%   +0.01%     
==========================================
  Files         219      219              
  Lines       10585    10586       +1     
==========================================
+ Hits        10177    10180       +3     
+ Misses        353      351       -2     
  Partials       55       55              

Impacted Files	Coverage Δ
cmd/collector/app/server/http.go	0.00% <0.00%> (ø)
cmd/collector/app/server/zipkin.go	0.00% <0.00%> (ø)
cmd/flags/service.go	0.00% <0.00%> (ø)
cmd/flags/admin.go	79.36% <83.33%> (ø)
cmd/collector/app/builder_flags.go	100.00% <100.00%> (ø)
cmd/collector/app/collector.go	69.11% <100.00%> (ø)
cmd/collector/app/server/grpc.go	65.38% <100.00%> (ø)
plugin/storage/badger/factory.go	98.14% <100.00%> (+0.01%)	⬆️
ports/ports.go	100.00% <100.00%> (+100.00%)	⬆️
... and 5 more

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[objectiser]

@burmanm Any concerns with this change?

[burmanm]

It's not really doing what I think the author thinks it's doing:

```// This is important. Without this Badger would keep atleast 1 version of very key.``

In reality, NumVersionsKeep is documented as: NumVersionsToKeep sets how many versions to keep per key at most. so it's a maximum, not minimum. Also, based on the code in the levels.go I don't see why it would make a difference in 1 vs 0:

https://github.com/dgraph-io/badger/blob/eaf64c0925f5a62d532986db526ed2c571cb22df/levels.go#L579

So if this helps with cleaning, it sounds like a bug in the badger rather than correct configuration option.

[mschneider82]

in dgraph-io/badger#1228 they recommend to set it to 0 to solve the issue, but that issue is still open, lets see how the default may change. I think instant drop is what we want for jaeger, so 0 is the setting for instant drop after expired (GC).

[CsterKuroi]

Looks nice to me.

[colinhoglund]

FYI, according to this closed PR (dgraph-io/badger#1300), changing this to 0 might have unintended effects on valid entries (dgraph-io/badger#1300 (comment)).

[jarifibrahim]

Hey! Please do not set number of versions to zero. Please see dgraph-io/badger#1228 (comment)