fix: implement staked-prefers-unvested policy

[JimLarson]

Description

Closes: Agoric/agoric-sdk#4579

Updates the behavior of PeriodicVestingAccount and ClawbackVestingAccount when receiving a new grant (or intercepted rewards in clawback accounts) to consistently interpret staked tokens as unvested first. Upon new grants, adjusts bookkeeping to recognize historical slashing to avoid premature access to tokens which should be locked. (See Agoric/agoric-sdk#4300)

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[michaelfig]

LGTM! Makes sense.