Save some time on restarting aborted indexing

[gilesknap]

Currently, because the mediaItem results are only delivered in reverse date order I do not save the 'most recently indexed' date unless a complete index has occurred (i.e. back to the earliest file in the photos library).

However, all subsequent scans could do the following to make sure all items are eventually scanned.

• Search the database for the earliest file and latest file
• Use the earliest file date to do a scan for all files before earliest
• Use the newest file date to do a scan for all files after the latest

This double scan approach negates the need for a completed scan before any incremental scans.
CAVEAT: I have noticed there is a difference between using mediaItems.list() and mediaItems.find() with no filters. For some reason, the latter misses a few (obscure format?) files.
CAVEAT2: I have also seen an issue with my library that a date range search always stops at 2010/09 (probably some strange meta on a photo)
TODO - try this approach and investigate how many files I miss in my photos library.

[gilesknap]

updated the description with an additional caveat. This is a low impact enhancement which is likely crippled by some weird behaviour in the API. Won't fix.