[Pools] Ensure members can always exit the pool gracefully (#4998)

Resolves https://github.com/paritytech-secops/srlabs_findings/issues/412

## Changes
- Clear any dangling delegation when member is removed.
- Agents need to be killed explicitly when pools are destroyed.
- Member withdraw amount is max of their locked funds and the value of
their points.

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Co-authored-by: Gonçalo Pestana <[email protected]>
Co-authored-by: command-bot <>