* Initial syncapi storage refactor to share pq/sqlite code
This goes down a different route than https://github.com/matrix-org/dendrite/pull/985
which tried to even reduce the boilerplate of `ExecContext` etc. The previous pattern
fails badly when there are subtle differences in parameters and hence the shared
boilerplate to read from `QueryContext` breaks. Rather than attacking it at that level,
the main place where we want to reuse code is for the `syncserver.go` itself - the
database implementation which has lots of complex logic. So instead, this commit:
- Makes `invites_table.go` an interface.
- Makes `SyncServerDatasource` use that interface
- This means some functions are now identical for pq/sqlite, so factor them out
to a temporary `shared.Database` struct which will grow until it replaces all of
`SyncServerDatasource`.
* Missing files
