Investigate fungible trait Holds and Freezes overlap.

[mattheworris]

As referenced here: Deprecate Currency; introduce holds and freezing into fungible traits
and implemented in the following PRs: #1779 #1818

The following comment indicates that we need to investigate some test cases where Holds are slashed and ensure that the affected pallets handle their bookkeeping for Freezes correctly:

Freezes are essentially the same as locks, except that they overlap with Holds. Since Holds are designed to be infallibly slashed, this means that any logic using a Freeze must handle the possibility of the frozen amount being reduced, potentially to zero. A permissionless function should be provided in order to allow bookkeeping to be updated in this instance.

Discussion

• Implement e2e tests that exercise the overlap between Holds and Freezes
  • Build a test where an account with a Freeze and a Hold and the Hold is slashed
  • Ensure the capacity pallet handles the logic correctly without any exploits
    • Check if amount staked is reduced to correlate with the amount slashed. Users should not be able to continue receiving the same capacity when tokens are slashed.
  • Ensure that a user can stake token to network and participate in governance with the same token amount (frozen amount).
  • Ensure the time-release pallet handles the logic correctly without any exploits

Changes

• Add e2e test with simultaneous Hold and Freeze.

How to Test

• Confirm that the new e2e tests pass.