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

1 files changed, 16 insertions, 0 deletions

diff --git a/syncapi/storage/tables/interface.go b/syncapi/storage/tables/interface.go
new file mode 100644
index 00000000..1a994052
--- /dev/null
+++ b/syncapi/storage/tables/interface.go
@@ -0,0 +1,16 @@
+package tables
+
+import (
+	"context"
+	"database/sql"
+
+	"github.com/matrix-org/dendrite/syncapi/types"
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+type Invites interface {
+	InsertInviteEvent(ctx context.Context, txn *sql.Tx, inviteEvent gomatrixserverlib.HeaderedEvent) (streamPos types.StreamPosition, err error)
+	DeleteInviteEvent(ctx context.Context, inviteEventID string) error
+	SelectInviteEventsInRange(ctx context.Context, txn *sql.Tx, targetUserID string, startPos, endPos types.StreamPosition) (map[string]gomatrixserverlib.HeaderedEvent, error)
+	SelectMaxInviteID(ctx context.Context, txn *sql.Tx) (id int64, err error)
+}