Enforce coinbase maturity in consensus rules

[alexdovzhanyn]

XEX is currently spendable as soon as it is mined. The issue with this instant allowance is that if a fork were to occur shortly after a block is generated, and the XEX generated in the coinbase of that block is spent, all transactions with that XEX would be invalidated (not delayed as what happens in a regular fork, as these transactions could never be valid because they are referencing a coinbase transaction that no longer exists). We need to define a coinbase maturation period which dictates how many blocks must be generated before a coinbase is deemed safe to spend. Bitcoin's coinbase maturation period is 101 blocks ~17 hours.

We should consider how many blocks a highly unlikely fork would constitute and base the maturation period from that. If we wanted to wait ~24 hours for coinbase transactions to be spendable we could set this threshold to 720 blocks.