Update, refine and document msc_initial_scan
Update, refine and document msc_initial_scan

- Increase time between progress reports from 15 s to 30 s
- Stop early, if a block could not be retrieved from the disk
- Extract progress reporting (to be ready to fire some signals or whatsoever)

OmniLayer/omnicore,OmniLayer/omnicore,OmniLayer/omnicore,OmniLayer/omnicore,OmniLayer/omnicore,OmniLayer/omnicore