Force KeepL0InMemory to be true when InMemory is true (#1375)
Force KeepL0InMemory to be true when InMemory is true (#1375)

InMemory mode would create L0 sst tables when KeepL0InMemory
was false as mentioned in #1374 This commit will keep L0 in
memory if either KeepL0InMemory is set or InMemory is set.

On running ` go test .`
Before: sst files were visible for a second 
Now: sst files won't created/visible

Fixes #1374
dgraph-io/badger,dgraph-io/badger