* upgrade aws roles

* test upgrade aws roles

* Initialize aws credential backend at mount time

* add a TODO

* create end-to-end test for builtin/credential/aws

* fix bug in initializer

* improve comments

* add Initialize() to logical.Backend

* use Initialize() in Core.enableCredentialInternal()

* use InitializeRequest to call Initialize()

* improve unit testing for framework.Backend

* call logical.Backend.Initialize() from all of the places that it needs to be called.

* implement backend.proto changes for logical.Backend.Initialize()

* persist current role storage version when upgrading aws roles

* format comments correctly

* improve comments

* use postUnseal funcs to initialize backends

* simplify test suite

* improve test suite

* simplify logic in aws role upgrade

* simplify aws credential initialization logic

* simplify logic in aws role upgrade

* use the core's activeContext for initialization

* refac...