* gas_limit for new blocks will divide evenly by 13
* increased PARITY_GAS_LIMIT_DETERMINANT to 37
* separate method for marking mined block
* debug_asserts(gas_limit within protocol range)
* round_block_gas_limit method is now static
* made round_block_gas_limit free-function
* multiplier->multiple
Former-commit-id: 247413e26cac28478ff47f85f754ff325903d8e9