Initial syncapi storage refactor to share pq/sqlite code (#1030)

* 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

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