rename geth_poa_middleware to something more generic

[dylanjw]

What was wrong?

It seems like many other chains have set higher length for extraData. see #898

How can it be fixed?

Should geth_poa_middleware be renamed to something like long_extraData_middleware? Or could the extraData length be configurable?

[carver]

Should geth_poa_middleware be renamed to something like long_extraData_middleware?

The geth poa middleware makes some assumptions about what the extraData is (naming it proof of authority).

Maybe something like:

def construct_move_extraData(rename_to='nonStandardExtraData'):
    ...

move_extraData_middleware = construct_move_extraData()

geth_poa_middleware = construct_move_extraData(rename_to='proofOfAuthorityData')

Or could the extraData length be configurable?

The middleware approach seems fine.