Add DNS Seeds and update checkpoint block hashes

[tenthirtyone]

This continues the work done by @kleetus

Changes:
Merges bcoin-org/bcoin master & resolves
Adds seed.bitcoinabc.org and Bitprim to DNS seeds
Updates checkpoint block hashes
Updates docs/Examples/fullnode.js network to bitcoincash - quick testing for now

This will get the network interface started and connect to dns seeds. We'll get new peers and start syncing as far as get headers. There's a promise error that isn't being handled, but this is a good starting point.

@nitsujlangston
@kleetus
@SonicWizard
@gasteve

in docs/Examples run
node fullnode.js

ER:

[info] (chain) Chain is loading.
[info] (chain) Checkpoints are enabled.
[info] (chaindb) Opening ChainDB...
[info] (chaindb) Writing genesis block to ChainDB.
[info] (chaindb) ChainDB successfully initialized.
[info] (chaindb) Chain State: hash=683e86bd5c6d110d91b94b97137ba6bfe02dbbdb8e3dff722a669b5d69d77af6 tx=1 coin=0 value=0.0.
[info] (chain) Chain Height: 0
[debug] (chain) Memory: rss=48mb, js-heap=15/21mb native-heap=26mb
[debug] (chain) Memory: rss=48mb, js-heap=15/21mb native-heap=26mb
[info] (mempool) Mempool loaded (maxsize=97656.25kb).
[info] (miner) Miner loaded (flags=mined by bcoin).
[warning] (miner) No reward address is set for miner!
[info] (net) Pool loaded (maxpeers=8).
[info] (net) Initialized header chain to height 0 (checkpoint=1d7c6eb2fd42f55925e92efad68b61edd22fba29fde8783df744e269000000000x).
[info] (http) Node HTTP server listening on 127.0.0.1 (port=8332).
[info] (node) Node is loaded.
[debug] (net) Connecting to 50.63.202.19:8333.
[info] (net) Adding loader peer (50.63.202.19:8333).
[debug] (net) Refilling peers (1/8).
[debug] (net) Connecting to 195.154.168.129:8333.
[info] (net) Connected to 195.154.168.129:8333.
[info] (net) Received version (195.154.168.129:8333): version=70015 height=494299 services=100101 agent=/Bitcoin ABC:0.15.1(EB8.0)/
[debug] (node) Added time data: samples=1, offset=33 (0 minutes).
[debug] (peer) Received verack (195.154.168.129:8333).
[debug] (peer) Version handshake complete (195.154.168.129:8333).
[info] (peer) Initializing normal compact blocks (195.154.168.129:8333).
[debug] (peer) Requesting headers packet from peer with getheaders (195.154.168.129:8333).
[debug] (peer) Sending getheaders (hash=683e86bd5c6d110d91b94b97137ba6bfe02dbbdb8e3dff722a669b5d69d77af6, stop=1d7c6eb2fd42f55925e92efad68b61edd22fba29fde8783df744e269000000000x).
(node:14515) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): AssertionError [ERR_ASSERTION]: false == true
(node:14515) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
[info] (peer) Peer initialized compact blocks (mode=0, version=1) (195.154.168.129:8333).
[info] (net) Received 1 addrs (hosts=2, peers=2) (195.154.168.129:8333).
[debug] (net) Refilling peers (2/8).
[info] (net) Received 1000 addrs (hosts=936, peers=2) (195.154.168.129:8333).