We expect to have missing events as we walk back in the DAG over federation
as we didn't always create the room. When checking if the server is allowed
to see those events, just give up and stop rather than fail the request.
* Add Queryer and use embedded structs
* Add Inputer and factor out more RS API stuff
This neatly splits up the RS API based on the functionality it provides,
whilst providing a useful place for code sharing via the `helpers` package.
- New package `perform` which contains all `Perform` functions
- New package `helpers` which contains helper functions used by both
perform and query/input functions.
- Perform invite/leave have no idea how to `WriteOutputEvents` and this
is now returned from `PerformInvite` or `PerformLeave` respectively.
Still to do:
- RSAPI is fed into the inviter/joiner/leaver - this introduces circular
logic so will need to be removed.
- Put query operations in a `query` package.
- Put input operations (and output) in an `input` package.
- Factor out helper functions as much as possible, possibly rejigging the
storage layer in the process.
