Consider alternatives to PersistentDict

[prjemian]

Use of bluesky's PersistentDict has been problematic at times, as described in these GitHub issues:

• Failure to recover RE.md from files on startup bluesky_training#300
• PersistentDict: Failure to recover RE.md from files on startup bluesky/bluesky#1741

Consider alternatives to this file-backed Python dictionary, which responsibility is to remember certain metadata keys (in RE.md) between sessions, such as scan_id.

[prjemian]

One possible alternative:
The GemViz (and MdaViz) projects have used Qt's QSettings to save application settings (window geometry, user-configurable choices, ...)

[prjemian]

Requirements to replace bluesky.utils.PersistentDict.

• Contents must be JSON serializable.
• Contents stored in a single human-readable file.
• Sync to file shortly after dictionary is updated.
• File format should support comments.
• Do not use zict. It uses a directory with one file per dictionary key. Zict creates an unnecessary problem (recurrent incidents at APS) with how it names these files. The problem is exacerbated when concurrent sessions are updating via PersistentDict.