This change prevents lingering shard and git-fast-import
processes from remaining after interrupted "lei import" (and
similar). It also reduces the likelyhood of data-loss in case
of subsequent abnormal termination of the daemon.
I think this is the least surprising way to handle users
prematurely aborting imports or other similar operations which
write to lei/store and will result in reduced bandwidth waste
for users with intermittent connections. This is because the
lei/store processes may be shared by parallel "lei import"
callers, and commits done by any "lei import" caller will
inevitably trigger writes for all of them.