Light client mode for simple Nodes (GRANDPA finality verification only)

[PaulSff]

Problem

While having Proof of Scan advanced logic to protect the PoW form ASIC/FPGA influence, it becomes difficult for simple nodes to sync.

Solution

The skip_check option allows for simple nodes to skip the blockchain seal and rely solely on GRANDPA finality.
ae6c732

To do

1. Test out the skip_check option and solve issues
2. Add security protection logic to prevent its usage while engaged with mining or running Node-validator