WIP: Improvements to on-demand indexing
This MR fixes tracker#122 (closed) and tracker#188.
The tracker index command now provides detailed feedback on indexing to the user. Rather than exiting as soon as the IndexFile method returns, it now waits until all the files have been processed and will report any errors to the user.
There are a few other tweaks. There is a tracker index --monitor mode added, which wraps the IndexFileForProcess API. Also, IndexFileForProcess now only accepts directory arguments, to avoid a confusing difference in its behaviour when files are passed compared to directories.
The main aim of this work is to make it easier to debug and profile the miner and extractor. However, it also paves the way for apps to provide more informative monitoring of the indexing process.