* 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
* Tackle #4929 a different way
This turns c.sealed into an atomic, which allows us to call sealInternal
without a lock. By doing so we can better control lock grabbing when a
condition causing the standby loop to get out of active happens. This
encapsulates that logic into two distinct pieces (although they could
be combined into one), and makes lock guarding more understandable.
* Re-add context canceling to the non-HA version of sealInternal
* Return explicitly after stopCh triggered
