Continuous Staking 1 - Introduce stakers update operation

[abi87]

Why this should be merged

Continuous staking is the upcoming feature that will allow long-term stakers to automatically restake past the specified staking period without signiging a new transaction. The object state.Staker is the in-memory representation of a staker in the platformvm. Currently state.Staker is an immutable object that can be added/deleted/queries, but not updated. Continuous staking requires the ability to:

• "shift a staker", that is to update state.Staker's time attributes (StartTIme/EndTime/NextTime).
• update staker weight, to allow reward compounding across different staking periods.

This PR unlock the possibility to do so.

How this works

First off, this PR is based off:

• https://github.com/ava-labs/avalanchego/pull/1510

Added UpdateCurrentValidator/Delegator operations to P-chain staker repositories. Refactored maskedIterator implementation to allow handling of updated stakers.

How this was tested

Some more property tests for stakers update operations. Some more property tests for validators set update following stakers operations. Highlighted different behavious in state.State and state.Diff with UTs Increased state package UT coverage to 52.3%

[github-actions[bot]]

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