Implement a simple fee system

[adrianbrink]

No description provided.

[grarco]

Quick implementation for testnets:

• Check balance in mempool_validate, reject tx if insufficient
• Check balance in process_proposal, reject tx if insufficient
• Check balance in finalize_block, reject tx if insufficient
• Burn fees in finalize_block (even if insufficient funds, drain whatever possible)
• Constant fee for every transaction (no protocol parameter to avoid the need for a chain restart)
• Fee can only be paid in NAM