Improve scanning process

[rigon]

The current scanning process is a bit clumsy.

I'm proposing a 3 tier scans. This will require an additional flag in Photo to indicate if a thumbnail was or was not generated.

1. Quick:
   • Look for new albums
   • Cache photos info
2. Regular (run by default):
   • Same as Quick, plus
   • Create all thumbnails for photos with the flag unset
3. Full:
   • This will verify if all data is correct, for that we need the following
   • Create a list of all thumbnails files stored
   • Load all albums
   • Scan all files in albums
   • Cache photos info
   • Create missing thumbnails
   • After finish processing an album:
     • Load all DB entries for that album and remove those not in the scanned list
     • Remove entries of exiting photos from the list of all thumbnails
   • In the end, the remaining entries in the list of all thumbnails should be removed
   • Entries in the DB for missing albums should be removed as well

Additionally, consider as well:

1. Creating a process to allow updating the cache in background. Benefits:
   • During the scan, do not to wait while updating the cache
   • Unify code where in some places is already updating cache in background
2. Allow suspend scan in the middle

[rigon]

Regular Scan runs by default when the server starts:

• No flag needed as runs by default
• To disable all scans, run with --disable-scan

Quick Scan is the same as Regular without creating thumbnails:

• To disable thumbnails during scans, run with --no-cache-thumbnails

Full Scan:

• Run with --full-scan
• The flag --no-cache-thumbnails can be used in conjunction