Incremental re-indexing instead of full re-indexes

[termermc]

Currently, shares are fully re-indexed periodically. That can be taxing on resources. Instead, use fsnotify to watch shares for file creation and deletion, and update the index accordingly. Manual re-indexing will still be possible, and it will still index on startup and share creation.

[termermc]

Also, indexes should be identified by path on disk, not by share UUID. The problem with the current system is that sharing the same path on multiple servers means it gets indexed multiple times.