Upstream added Lightning constants

[JeremyRand]

Upstream added the following to constants.py:

    BLOCK_HEIGHT_FIRST_LIGHTNING_CHANNELS = 497000
    LN_REALM_BYTE = 0
    LN_DNS_SEEDS = [
        'nodes.lightning.directory.',
        'lseed.bitcoinstats.com.',
    ]

We need to look into rebranding them for Namecoin.