gas_limit for blocks, mined by Parity will be divisible by 37 (#4154) (#4176)

* 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 renamed to multiple