The primary use case for this is currently the DataAPI, and the quote id is
only available at the highest level of the server, before dapis are
processed.
In any case, the TokenStore was already described in terms of a combination
of document id; namespace; and root field; so it makes sense for doc id to
be part of the constructor.
If a more generic TokenStore is needed in the future, we could go back to
the previous API and wrap it in another class, like a partially applied
function (e.g. `DocumentTokenStore`).
* src/server/token/TokenStore.ts: Move doc_id out of arguments and into the
constructor.
* test/server/token/TokenStoreTest.ts: Update accordingly.
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