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

syncapi/storage/shared/syncserver.go

@@ -0,0 +1,37 @@
+package shared
+
+import (
+	"context"
+	"database/sql"
+
+	"github.com/matrix-org/dendrite/common"
+	"github.com/matrix-org/dendrite/syncapi/storage/tables"
+	"github.com/matrix-org/dendrite/syncapi/types"
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+// Database is a temporary struct until we have made syncserver.go the same for both pq/sqlite
+// For now this contains the shared functions
+type Database struct {
+	DB      *sql.DB
+	Invites tables.Invites
+}
+
+func (d *Database) AddInviteEvent(
+	ctx context.Context, inviteEvent gomatrixserverlib.HeaderedEvent,
+) (sp types.StreamPosition, err error) {
+	err = common.WithTransaction(d.DB, func(txn *sql.Tx) error {
+		sp, err = d.Invites.InsertInviteEvent(ctx, txn, inviteEvent)
+		return err
+	})
+	return
+}
+
+func (d *Database) RetireInviteEvent(
+	ctx context.Context, inviteEventID string,
+) error {
+	// TODO: Record that invite has been retired in a stream so that we can
+	// notify the user in an incremental sync.
+	err := d.Invites.DeleteInviteEvent(ctx, inviteEventID)
+	return err
+}