add MPI_File_fence

[adammoody]

@roblatham00 , I was hacking together a prototype for MPI_File_fence. We talked about defining two more calls: MPI_File_flush and MPI_File_fetch, but I'll just stick with fence to start with. I think fence is likely the call HDF would eventually use in place of:

MPI_File_sync()
MPI_Barrier()
MPI_File_sync()

Recall the fence synchronizes with the file system servers to expose newly written data, but it does not require the data to be flushed to disk. The data only need to be made visible for reads by other procs. Fence also implies a global barrier across ranks so that all ranks know that all other ranks have also flushed their data upon returning from fence.

One intrusive change is that it requires adding a new field to the struct of ADIO function pointers. I've not completed that for all ADIO implementations. I just added it in a couple places to bring up the point.

[adammoody]

With the second commit in this PR, this at least now compiles for me. It's still not added to the function pointer table across all ADIO implementations yet. It's now defined for NFS, TESTFS, UFS, Lustre, GPFS, and UNIFY.

[adammoody]

I extended the src/mpi/romio/test/perf.c test as a check and verified that it works on NFS.

[adammoody]

@roblatham00 , for POSIX-compliant file systems like Lustre and GPFS, the MPI_File_fence only needs to execute a barrier. That's how I implemented the GEN_Fence.

However, for file systems that are not POSIX-compliant like NFS, ADIO_Fence may need to do more, perhaps even fall back to call ADIO_Flush for an fsync. I could use your help in adding it to the other backends.