Instead of getting batches of unused primary key ids, even if they're lower
than other ids, which for some reason seemed like a good idea in 2008, just do
a `MAX()` on the table at startup and return the next available id on each call
to `Zotero.ID.get()`. This is much simpler, and not reusing ids allows them to
be used as a chronological sort field.
While SQLite's `SELECT last_insert_rowid()` could return auto-increment values,
it's unsafe with async DB access, since a second `INSERT` can come in before
the first `last_insert_rowid()` is called. This is true even in a transaction
unless a function that calls it is never called in parallel (e.g., with
`Zotero.Promise.all()`, which can be faster than sequential `yield`s).
Note that the next id is always initialized as MAX() + 1, so if an object is
added and then deleted, after a restart the same id will be given. (This is
equivalent to (though unrelated to) SQLite's `INTEGER PRIMARY KEY` behavior,
as opposed to its `INTEGER PRIMARY KEY AUTOINCREMENT` behavior.)
Closes#993, Feed items out of order
fabc2ba6a2