Reconfigurable Swapper

[muhlemmer]

Make Swapper reconfigurable so that the Hasher can be changed during runtime and Verifier added.

Acceptance criteria

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1. Add a RWMutex to Swapper for safe concurrent access
2. Add SetHasher() method to Swapper
3. Add AddVerifier() method to Swapper
4. When the new Hasher is unable to verify passwords from the old Hasher, add the old Hasher to the Verifier stack. (might require additional methods on Verifier interface, TBD)

Note, deleting Verifiers doesn't make sense and can only lead to problems and should therefore not be supported.

[muhlemmer]

closing as obsolete, configuration is done by the caller, in our case Zitadel.