Move Fee Adjustment Logic into new pallet

[gabriele-0201]

After what was done in #161 and what was described here, all the Fee Adjustment Logic should be refactored into a new pallet.

This refactor would then require a small change in the runtime/lib.rs to start using the new fee adjustment logic contained in the pallet

[gabriele-0201]

The StorageValue in the pallet will be:

• NextLengthMultiplier :: updated in each on_finalize
• TargetBlockSize :: Having this parameter in storage could give the chain the opportunity to adjust itself if there is high demand

But what about TargetBlockFullness? Having this on-chain as a storage value would give the same flexibility as TargetBlockSize to change over time. However, the fullness of each block, in terms of weight and proof size, should generally remain constant. Do you agree to keep it as a constant in the pallet Config?

[gabriele-0201]

But what about TargetBlockFullness? Having this on-chain as a storage value would give the same flexibility as TargetBlockSize to change over time. However, the fullness of each block, in terms of weight and proof size, should generally remain constant. Do you agree to keep it as a constant in the pallet Config?

This is no longer a problem. The length-adjustment fee is currently decoupled by the TargetedFeeAdjustment logic, so TargetBlockFullness is not stored in the pallet