IMPALA-1214: Fix missing lock in BufferdBlockMgr.

GetUnusedBlock is not thread-safe and needs to be called with a lock taken.

Change-Id: Idaa4e2f49e27e0d01653cc7e62f9ebc7317fe5ec
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4223
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: Nong Li <nong@cloudera.com>