Library folder listing decrypts every file in the folder #445

Problem

Listing a Library folder fully decrypts every protected file in it. library.rs:1266 calls into :3411 for each item, under the global object lock. A folder with 40 files of 100 MiB (the upload limit) means about 4 GiB of decryption for every listing. This is an estimate, not measured.

Found while designing #444 (local://444-design-v2.md, open point 2), at bfefe92. Outside #444's requirements.

Required behavior

• Listing a folder costs work proportional to the number of items, not their bytes: metadata from the envelope header or a cached summary, no full decryption.
• Opening, previewing or downloading a file still fully authenticates it before use (Home cannot start, update or show Recovery with a file over 16 MiB (or 64 MiB total) in Library folders #444).
• Installed measurement: listing time for a folder with 40 files of 100 MiB, on Mac and Linux.

Milestone

After 0.8.0, unless the #444 installed journey 2 measures listing time that makes Library unusable with ordinary media. In that case this moves to 0.8.0.