locker: fix race in locking
locker: fix race in locking

The isolation level is lower than I thought. This made it possible for
two clients to think they both locked the same machines, since the
update would still be modifying each row to change the locked_since
time.

SUSE/teuthology,tchaikov/teuthology,michaelsevilla/teuthology,SUSE/teuthology,michaelsevilla/teuthology,zhouyuan/teuthology,robbat2/teuthology,tchaikov/teuthology,dmick/teuthology,ivotron/teuthology,t-miyamae/teuthology,yghannam/teuthology,ceph/teuthology,dreamhost/teuthology,caibo2014/teuthology,ktdreyer/teuthology,t-miyamae/teuthology,dmick/teuthology,SUSE/teuthology,ktdreyer/teuthology,robbat2/teuthology,dreamhost/teuthology,caibo2014/teuthology,ivotron/teuthology,yghannam/teuthology,ceph/teuthology,dmick/teuthology,zhouyuan/teuthology