hooks
doing something like this:
err = provider.updateUser(u)
...
return provider.userExists(username)
could be racy if another update happen before
provider.userExists(username)
also pass a pointer to updateUser so if the user is modified inside
"validateUser" we can just return the modified user without do a new
query
The common package defines the interfaces that a protocol must implement
and contain code that can be shared among supported protocols.
This way should be easier to support new protocols
