* Move seal barrier type field from Access to autoSeal struct.
Remove method Access.SetType(), which was only being used by a single test, and
which can use the name option of NewTestSeal() to specify the type.
* Change method signatures of Access to match those of Wrapper.
* Turn seal.Access struct into an interface.
* Tweak Access implementation.
Change `access` struct to have a field of type wrapping.Wrapper, rather than
extending it.
* Add method Seal.GetShamirWrapper().
Add method Seal.GetShamirWrapper() for use by code that need to perform
Shamir-specific operations.
* OSS portion of wrapper-v2
* Prefetch barrier type to avoid encountering an error in the simple BarrierType() getter
* Rename the OveriddenType to WrapperType and use it for the barrier type prefetch
* Fix unit test
Shamir seals now come in two varieties: legacy and new-style. Legacy
Shamir is automatically converted to new-style when a rekey operation
is performed. All new Vault initializations using Shamir are new-style.
New-style Shamir writes an encrypted master key to storage, just like
AutoUnseal. The stored master key is encrypted using the shared key that
is split via Shamir's algorithm. Thus when unsealing, we take the key
fragments given, combine them into a Key-Encryption-Key, and use that
to decrypt the master key on disk. Then the master key is used to read
the keyring that decrypts the barrier.
