BEP-524: short block interval phase two: 0.75 seconds

[zzzckck]

Description

There would be 2 phases to shorten BSC block interval from 3 seconds to 0.75 seconds:

• Phase 1: 3s -> 1.5s
• Phase 2: 1.5s -> 0.75s

TwoPhase strategy is to avoid one big step to reduce it to 0.75s directly, which could be too risky, as the overall infrastructure may not be fully ready to this change.
And it also update some parameters of Phase-1:

• Epoch: 2000 -> 1000, too long epoch would be not good to update the validator set timely
• GasLimit: simply prefer 60M over 65M
• Blob Update Fraction: it actually have very limited impact to BSC blob gas price, prefer to remove it for simplicity.
• TurnLength: some users may be very cautiously toward TX finalization, they would wait for (2/3)Validators before confirm the transaction. It would be (2/3)21321.5s = 672 seconds, which is too long.