Deadlock when calling App::current_user from a User listener

[kraenhansen]

When calling App::current_user from a listener callback registered on a User the thread deadlocks while waiting for the m_user_mutex to release:

realm-core/src/realm/object-store/sync/sync_manager.cpp

Lines 390 to 392 in e593a5f

	std::shared_ptr<SyncUser> SyncManager::get_current_user() const
	{
	util::CheckedLockGuard lock(m_user_mutex);

But this was already taken by the calling code, here:

realm-core/src/realm/object-store/sync/sync_manager.cpp

Lines 339 to 342 in e593a5f

	std::shared_ptr<SyncUser> SyncManager::get_user(const std::string& user_id, const std::string& refresh_token,
	const std::string& access_token, const std::string& device_id)
	{
	util::CheckedLockGuard lock(m_user_mutex);

This might be related to 64bf842 which was merged 3 months ago and introduced the call to user->log_in:

realm-core/src/realm/object-store/sync/sync_manager.cpp

Line 361 in e593a5f

user->log_in(access_token, refresh_token);

This shows a minimal reproduction in the Realm JS integration tests:

https://github.com/realm/realm-js/blob/f06739da3bcc77dce8e338b1f8960644a33be2c8/integration-tests/tests/src/tests/sync/user-listener-lockup.ts#L31-L43