Set default value of NumVersionsToKeep to 0

[jarifibrahim]

Issue #1228 showed that badger doesn't clean SST files even if all the keys are deleted. This is the expected behavior if NumVersionsToKeep is set to 1 but for the users this is counter-intuitive. When a key is deleted, it should be removed from the system. This PR fixes it.

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This change is 

[ou05020]

@jarifibrahim Doesn't this enable infinite versions? lastValidVersion will never be true because numVersions will never be less than 1 when compared to NumVersionsToKeep. Without lastValidVersion=true the skipKey will never get set, allowing all versions of a key to get written.

https://github.com/dgraph-io/badger/blob/master/levels.go#L579-L580

[jarifibrahim]

@ou05020 Yes. You are correct. I was looking at only the deleted entries and forgot how this would affect the valid entries. Thank you so much for looking at it :)

Closing this PR.