Resource tags optimistic locking

[lemon24]

Some kind of locking is probably required to implement a use case like reactions (thumbs up/down etc., #228 (comment)) correctly. Use cases like implementing comments (with versions) might benefit too.

Optimistic locking seems like a reasonable enough solution.

(Note we don't need this for feeds/entries because they're more or less eventually consistent.)

Relevant resources:

• https://docs.peewee-orm.com/en/latest/peewee/hacks.html#optimistic-locking

Open questions:

• What does the API look like?
  • Can we use a reserved top level key?
    • I seem OK with .reader.search in Entry tags and metadata #228 (comment).
  • OTOH, reserved keys are for end-user-facing features...
    • But, is it reader's responsibility to control how they are shown? Presumably, for a use case like the one described above, the user would not see those tags directly, but see them rendered as 👍, 👎 anyway.