bugfix: Fix a race condition when creating guest accounts (#882)

* bugfix: Fix a race condition when creating guest accounts

It was possible to both select the same next numeric ID and then both
attempt to INSERT this into the table. This would cause a UNIQUE violation
which then presented itself as an error in sqlite because it does not
implement `common.IsUniqueConstraintViolationErr`.

The fix here is NOT to implement `common.IsUniqueConstraintViolationErr`
otherwise the 2 users would get the SAME guest account. Instead, all of
these operations should be done inside a transaction. This is what this
PR does.

* Update postgres

* Typo

* Actually use the txn when creating accounts

* bugfix for database is locked on guest reg

clientapi/auth/storage/accounts/postgres/account_data_table.go

@@ -72,9 +72,9 @@ func (s *accountDataStatements) prepare(db *sql.DB) (err error) {
 }
 
 func (s *accountDataStatements) insertAccountData(
-	ctx context.Context, localpart, roomID, dataType, content string,
+	ctx context.Context, txn *sql.Tx, localpart, roomID, dataType, content string,
 ) (err error) {
-	stmt := s.insertAccountDataStmt
+	stmt := txn.Stmt(s.insertAccountDataStmt)
 	_, err = stmt.ExecContext(ctx, localpart, roomID, dataType, content)
 	return
 }