[ETCM-426] Changed the gas limit strategy to a constant one

[ntallar]

Description

Our current strategy is to always increase the gas limit of our blocks as much as possible which in our current cluster setup resulted in:

• Exposed a bug on web3, as that big a number was being properly shown by the explorer
• Means that after 20k blocks 900k million txs could be sent per block, which is very attackable

Proposed Solution

• The genesis gas limit was set to the ETC mainnet value: 8 million
• The miners strategy was changed to always set the gas limit of blocks to the same as the parent

For future tasks

A proper voting strategy (with a target as other clients) should be set up

Testing

Start up the node and mine a couple blocks, check that all the blocks have a gasLimit of 8 million

[mirkoAlic]

LGTM!