Multiple servers start up and execute as independent single-task jobs

Nobody has claimed this yet.

Hit a corner case that took a while to debug. Since I like to run under totalview, I still prefer to launch the servers by hand. That looks something like this hacky thing:
```
procs=$SLURM_NNODES
srun -n $procs -N $procs touch /var/tmp/unifyfs.conf
srun -n $procs -N $procs mkdir /dev/shm/unifyfs

export UNIFYFS_CONFIGFILE=/var/tmp/unifyfs.conf
export UNIFYFS_SERVER_LOCAL_EXTENTS=0
export UNIFYFS_SHAREDFS_DIR=/p/lustre2/user1
export UNIFYFS_DAEMONIZE=off

totalview srun -a -n $procs -N $procs `pwd`/bin/unifyfsd
```
However, in this case, I apparently built without PMI support, and I had forgotten to create a servers hostfile. In that situation each server initializes ```glb_pmi_rank = 0``` and ```glb_pmi_size = 1```. During the run, each server thinks it is server rank 0, and so it generates key/value pairs using that information.

Then I ran a job with two nodes, one server per node, and one client per node. Each client wrote one extent, and the extents do not overlap. Each server inserted one key/value pair into MDHIM while both assumed they were server rank=0. When reading the data back out, a server ends up reading back both keys from MDHIM successfully, but then it thinks both extents are local, since ```delegator_rank=0``` in both key/value pairs. This results in a file that is the correct size, but the read returns "corrupted" data.

Not sure what to do about it yet, but I wanted to document the case to capture the details. Maybe we can make the server host file be a required setting when PMI isn't being used?

Also, what happens if PMI is enabled but then one sets a hostfile?

I did verify things worked again when I created the servers hostfile, which I did with this other hack:
```
export UNIFYFS_SERVER_HOSTFILE=/p/lustre2/user1/unifyfs_server_hosts
rm -f $UNIFYFS_SERVER_HOSTFILE
echo $SLURM_NNODES > $UNIFYFS_SERVER_HOSTFILE
srun -n $SLURM_NNODES -N $SLURM_NNODES /bin/hostname >> $UNIFYFS_SERVER_HOSTFILE
```

Start at the bin/unifyfsd launch path and inspect how PMI state and UNIFYFS_SERVER_HOSTFILE are handled. Reproduce the two-node, one-server-per-node case with PMI absent, then compare it with the hostfile setup; done means server ranks and delegator ranks are distinct and the resulting extents are read correctly.

Written by the indexing model from the issue text.