Review transaction fees in Gear

[ekovalev]

A provisional implementation of transaction fee system for Gear has been implemented (pallet-gear-payment). However, it has a few drawbacks we need to get back to after we've got more usage statistics of the current implementation on the testnet, including (but not necessarily limited to):

• extrinsics gas limit is not taken into consideration;
• the fee multiplier changes exponentially with respect to the message queue length which can lead to extremely high fees;
• there is some rounding error due to the current algorithm that may alter the fee amount in some cases; for instance, a lot of messages in message queue, a call that doesn't modify the message queue and the extrinsic size is relatively small - in this case the effect of rounding can be noticeable;
• eventually, a floating threshold that splits the block time between extrinsics processing and messages execution will be introduced; that change will affect the way fee multiplier is calculated.