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

db.go

@@ -218,6 +218,8 @@ func Open(opt Options) (db *DB, err error) {
 	if opt.Compression == options.ZSTD && !y.CgoEnabled {
 		return nil, y.ErrZstdCgo
 	}
+	// Keep L0 in memory if either KeepL0InMemory is set or if InMemory is set.
+	opt.KeepL0InMemory = opt.KeepL0InMemory || opt.InMemory
 
 	// Compact L0 on close if either it is set or if KeepL0InMemory is set. When
 	// keepL0InMemory is set we need to compact L0 on close otherwise we might lose data.