Proto/AI: check impact of full unstake request in apply_delegation (Follow-up from "Proto: request a full unstake on set_delegate")

apply_delegation now performs a full unstake request, both for internal or external operations.

• Gas consumption must be taken into account.

But we don't want delegation to become costly:

• how much a flat rate would cost?
• do we want to be more precise? (how precise? is 0/1 OK?)

Since smart contracts cannot costake (yet),

• the gas impact for them should be close to zero
• should we add shortcuts? (check contract kind)

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The following discussion from !9033 (merged) should be addressed:

• @mbouaziz started a discussion:

  In %Adaptive Inflation - Stabilization: check impact on gas for delegation