Fix broken lastAuthUser lookup in CognitoUser

[malkovro]

Why?

A refactor as part of #130 shifted a bit too much of code. It led to the change of key of the LastAuthUser attribute of the CognitoUser class from CognitoIdentityServiceProvider.${pool.getClientId()}.LastAuthUser to CognitoIdentityServiceProvider.${pool.getClientId()}.$username.LastAuthUser.

On the User Pool class, the lastAuthUser is still looked up under the previous key...

Changes

• Add a far from elegant poolKeyPrefix used only for the LastAuthUser in CognitoUser class
• Rewrite the keyPrefix as the poolKeyPrefix.$username

[dgofman]

https://github.com/furaiev/amazon-cognito-identity-dart-2/pull/140

Should fix that issue. There are two problems

[furaiev]

Closed by https://github.com/furaiev/amazon-cognito-identity-dart-2/pull/140