This change addresses several points:
1. The integration tests didn't verify that delegations could be used
for decryption following a restore. The integration tests now
verify this.
2. There was no functionality for clearing persisted delegations if
needed. The vault admin can now do this via the command line tool.
3. Restoring active delegations wasn't storing the key with the
delegation. Keys are now serialised properly.
4. [Minor] The MSP package now reports the name of the offending user
when it can't find a user name in the database.
+ persistLabels moved from cryptor to persist package global.
+ Restore now explicitly checks for the case where there aren't enough
shares to return `ErrRestoreDelegations`.
+ The users responsible for restoring the cache are now logged.
This is a rather large change. It consists of the following changes:
+ Direct access to the keycache has been removed from the core
package. This forces all interaction with the cache to go
through the Cryptor, which is required for persistence. The
Cryptor needs to know when the cache has changed, and the only
way to do this effectively is to make the Cryptor responsible
for managing the keycache.
+ A new persist package has been added. This provides a Store
interface, for which two implementations are provided. The
first is a null persister: this is used when no persistence
is configured. The second is a file-backed persistence store.
+ The Cryptor now persists the cache every time it changes.
Additionally, a number of missing returns in a function in the core
package have been added.
