SEC-34 Add check for minGasLimit in new gas limit validation

[Gustav-Simonsson]

minGasLimit should be read from https://github.com/ethereum/common/blob/master/params.json and verified in the new gas limit validation at https://github.com/ethereum/go-ethereum/blob/develop/core/block_processor.go#L263

Otherwise there if gas limit becomes less than 1024 it can never be changed again due to the new validation rules.

[obscuren]

#594

[Gustav-Simonsson]

Added: https://github.com/ethereum/go-ethereum/blob/develop/core/block_processor.go#L271