I removed the `readonlyGqlPostHandler` thingy from airgql,
as it seemed to be a slightly worse version of the existing more
general gql post handler. The one difference is that it was trying
to use the readonly db directory, although the token already protected
the queries to reading only, so this didn't matter. Moreover, the
writeonly endpoint was also using the readonly directory, which seemed
like a mistake (I removed that handler as well in favour of passing the
`writeonly` access mode to the existing handler).
I also fixed a TODO, making it so the readonly airsequel-cli handler
takes the edition into account.
So far we were going off the primary_key flag given to us by sqlite, but
this flag is only set to true for one column when using multi-column
primary keys.
Moreover, this commit also implements a `isRowid` property on
`ColumnEntry`, which is both there for convenience, and to reuse the
"proper" (i.e., taking into account rowid columns named _rowid_) rowid
detection logic.
