fix attempt to sync unlinked file

[MichaelBrim]

Description

• Adds mutual exclusion locking of unifyfs_client state to prevent client threads from seeing inconsistent state.
• Fixes a bug where unifyfs_unmount() would fail if an unlink client callback was processed during an attempt to sync extents for the removed file.

Motivation and Context

Running the writeread example on Summit with the option to remove the target file at completion was causing unifyfs_unmount() failures in most clients.

For example:

2021-09-01T15:09:53 tid=1297965 @ unifyfs_fid_sync_extents() [unifyfs_fid.c:1057] missing filemeta for fid=1
2021-09-01T15:09:53 tid=1297965 @ unifyfs_finalize() [unifyfs_api.c:253] client sync failed
UNIFYFS ERROR: unifyfs_unmount() failed with 'Failure'

How Has This Been Tested?

Tested on OLCF Summit and in Ubuntu Docker.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Performance enhancement (non-breaking change which improves efficiency)
• [ ] Code cleanup (non-breaking change which makes code smaller or more readable)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Testing (addition of new tests or update to current tests)
• [ ] Documentation (a change to man pages or other documentation)

Checklist:

• [x] My code follows the UnifyFS code style requirements.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.
• [x] All commit messages are properly formatted.

[MichaelBrim]

@adammoody please look over and merge asap