Register race condition: User ID in use

[dbkr]

The registration endpoint uses set_session_data (https://github.com/matrix-org/synapse/blob/master/synapse/rest/client/v2_alpha/register.py#L486) and get_session_data (https://github.com/matrix-org/synapse/blob/master/synapse/rest/client/v2_alpha/register.py#L326) on the auth_handler to remember that it has registered a user for the UI auth session and return the same user on subsequent calls for idempotency.

Unfortunately there is enough code between these calls that when synapse is having a slow day, another rquest can come in and get past the get_session_data before the first has hit set_session_data, in which case one (some) of the requests return the User ID in use error rather than returning the same session.