While over-estimation fees and miner tips are still paid, gas is no longer burnt
for direct, successful window PoSt messages.
Usually, gas is burnt to prevent an attacker from spamming the network and to
allow clients to "price" messages (using the base fee cap) based on how urgently
they need them to be processed. However:
1. Window PoSt is already a "proof of work".
2. Miners need to submit WindowedPoSts on-time so all window post messages are urgent.
3. Work is already under way to move window post verification off-chain (making
it effectively free). This change simply introduces the "free" part a bit earlier.
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