Fixing SyncedFileSystemDirectoryFactory when the main index is corrupted

[Shazwazza]

Looking at stack traces reported from SyncedFileSystemDirectoryFactory shows that the errors always come from when the directory is first opened (i.e. site startup).

This adds an index checker and the ability to try to repair the index. The check will happen before the index is attempted to be open. If the check fails, it will try to fix it and report on the outcome. If a fix occurs, this generally means that documents may be lost, however, they might have been removed documents in the first place but the segments file wasn't updated during the syncing.

The index will then attempted to be read, if that fails, a new index will be created.

• If the main index is unhealthy, check the local index, if it is healthy then eagerly sync back to main before initializing.
• If the main index is unhealthy and no way to restore from local, do we want to 'fix' it which may end up removing documents? In some cases, the documents that will be removed, should be removed since this would have resulted from syncing deleted files but not have synced the segments file correctly. Alternatively, we would just clear the index and a rebuild would be needed (forced by Umbraco for example)