Allow all version of keys in key only iteration

The key only iterator is supposed to iterate over all versions of a
given key. The current implementation didn't enable all version of the
same key by default and the user had to manually do it. This commit sets
the all versions flag to true by default.

iterator.go

@@ -435,6 +435,7 @@ func (txn *Txn) NewKeyIterator(key []byte, opt IteratorOptions) *Iterator {
 	}
 	opt.Prefix = key // This key must be without the timestamp.
 	opt.prefixIsKey = true
+	opt.AllVersions = true
 	return txn.NewIterator(opt)
 }
 
@@ -459,6 +460,9 @@ func (it *Iterator) Valid() bool {
 	if it.item == nil {
 		return false
 	}
+	if it.opt.prefixIsKey {
+		return bytes.Equal(it.item.key, it.opt.Prefix)
+	}
 	return bytes.HasPrefix(it.item.key, it.opt.Prefix)
 }