Properly restore delegations.

[kisom]

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.

[codecov-io]

Current coverage is 32.24% (diff: 29.82%)

Merging #170 into master will increase coverage by 0.05%

@@             master       #170   diff @@
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  Files            21         21          
  Lines          4274       4329    +55   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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+ Hits           1376       1396    +20   
- Misses         2673       2699    +26   
- Partials        225        234     +9   

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[grittygrease]

LGTM, though some functions (ResetPersisted) aren't covered by tests.

[kisom]

I've added some more tests.

[lziest]

lgtm