* 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
* sql/backwards_extremities: Shift to table format and share code
This is an initial cut to reduce boilerplate at the storage layer.
It removes the need for 2x `_table.go` files, one for each DB engine,
replacing it with a single struct which has an interface which
implements the raw SQL statements.
The actual impl sits alongside the interface declaration which is
generally regarded as best practice (though no canonical sources).
Especially in this case where the impl is tiny (functions returning
strings) and relies heavily on the function signatures of the
table struct (for parameters), having the context in the same file
is useful.
* Remove _table redundancy
