WIP: Enhanced Deletion

[samdeane]

Work in progress on some changes for use in Agenda.

Adds the facility to replace deleted blob files with a "tombstone" marker file which forces the rest of the API to treat the blob as deleted even if the actual data file returns.

This is intended to work around situations where the blob directory is being synced with an external service that might get confused and put back deleted files instead of removing them.

Note: the behaviour of the existing deleteBlobAtPath call should be unaffected by these changes. A new variant of the call has been added to opt-in to tombstones. Other API has been updated to be aware of the presence of tombstone files, but should be unaffected if they are never created. There will be a minor reduction in efficiency due to the need to check for tombstones, but it shouldn't be significant in comparison to whatever work is actually done with the files.