Support for switch/fork to override epochSize

[dankostiuk]

I know the epochSize is currently defined on genesis initialization directly within the ibft object or 100000 by default -- however I think it would be beneficial for chain operators to request their validators to apply a fork/switch to override the genesis epochSize with a new value in case it is decided later on that the chain would benefit from some other sort of epochSize value..

For example the corresponding ibft object within the genesis.json could look something like this:

 "engine": {
            "ibft": {
                "quorumSizeBlockNum": 5,
                "types": [
                    {
                        "type": "PoA",
                        "from": "0x0",
                        "to": "0xc7",
                        "epochSize": 100000
                    },
                    {
                        "type": "PoA",
                        "from": "0xc8",
                        "to": "0x12b",
                        "epochSize": 100
                    },

In our case, our chain defaulted to the epochSize of 100000 ~ which is around 55 hours assuming 2 second block times.

We have some custom logic to commit transitions at the end of each epoch which we wish to occur on a smaller time interval than 55 hours so we'd be happy to create a PR for this as I think it would be beneficial for others as well...

Thoughts?