Update SQLConf.scala

use concurrent.ConcurrentHashMap instead of util.Collections.synchronizedMap
This follows code style from `BlockManager`:

      def getLocalFromDisk(blockId: BlockId, serializer: Serializer): Option[Iterator[Any]] = {
        diskStore.getValues(blockId, serializer).orElse(
          sys.error("Block " + blockId + " not found on disk, though it should be"))
      }
Anyway `throw` expression return `Nothing`, which can work with both `getOrElse` and `orElse`