* Use federation sender for backfill and getting missing events
* Fix internal URL paths
* Update go.mod/go.sum for matrix-org/gomatrixserverlib#218
* Add missing server implementations in HTTP interface
- 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.
