Use time instead of block for SGT activation

[blockchaindevsh]

For context, refer to #13 (comment) .

[blockchaindevsh]

@qizhou @qzhodl when looking into it, I found that in op-geth there's no known L2BlockTime(which is 2s most of the time). It's available in op-node here, but sadly not available for op-geth.

L2BlockTime is needed in these call sites if we want to switch from block to time:

op-geth/core/txpool/blobpool/blobpool.go

Line 683 in bbc7290

balance, sgtBalance := core.GetGasBalances(p.state, p.chain.Config(), addr, p.head.Number.Uint64()+1)

op-geth/core/txpool/legacypool/legacypool.go

Line 1494 in bbc7290

balance, sgtBalance := core.GetGasBalances(pool.currentState, pool.chainconfig, addr, pool.currentHead.Load().Number.Uint64()+1)

op-geth/core/txpool/legacypool/legacypool.go

Line 1685 in bbc7290

balance, sgtBalance := core.GetGasBalances(pool.currentState, pool.chainconfig, addr, pool.currentHead.Load().Number.Uint64()+1)