Add client unlink callbacks

[MichaelBrim]

Description

This callback informs clients that they should cleanup any associated file state, including releasing storage for unsynced extents. The callback rpc is only sent to clients that have previously registered extents with their local server.

The client callback support in the request manager thread has been designed to make it easy to add other callbacks in the future.

Also:

• adds an integer flag to the logio header to avoid concurrent updates by the client and server.
• adds a program option (-u|--unlink) to examples to unlink target file
• adds a new arraylist_sort() method

Motivation and Context

Fixes a few of the deficiencies identified in review of PR #656

How Has This Been Tested?

Tested in Ubuntu Docker and on OLCF Summit

Types of changes

• 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:

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

[adammoody]

Nice work, @MichaelBrim . Wow, that's a lot of new stuff, and the new general support for client rpcs will be useful.