ReplayGain: import stage write callbacks can have outdated metadata

[ybnd]

Problem

After parallel ReplayGain analysis on import, writing to newly imported files can fail if they're moved.

Temporary solution

• Disable replaygain parallelism during import #3818
• The results are still stored in the database, so you can persist them to the imported files by running beet write after importing.

Setup

• OS: any
• Python version: any
• beets version: 206b1c1
• Turning off plugins made problem go away (yes/no): N/A

Configuration:

plugins: replaygain ...

replaygain:
   auto: yes
   overwrite: yes