Add --ramdb option to osxphotos exportdb --migrate-photos-library

[RhetTbull]

Thanks a lot @RhetTbull ! Impressive!
I launched the script already since yesterday but it seems it will take a long time. I have 140000 assets in the Library and it seems that it's taking around 10 seconds per asset to recognize it.
If I'm not wrong it will take 16 days to finalize this.
Perhaps you have an idea on how can I speed it up?

The OSXPhotos backup is on a SSD Drive on a Synology NAS
The Library is on an External Hard Drive connected to an iMac M3 with 16GB Ram.

If not, perhaps, it will be better to reexport everything from the new Library

Originally posted by @jeremieberrebi in #1415 (comment)