Add Testnet4 support

[thecodefactory]

Depends on libbitcoin/libbitcoin-system#1866 and libbitcoin/libbitcoin-node#1034

UPDATE: Also depends on libbitcoin/libbitcoin-database#820
UPDATE: Also depends on libbitcoin/libbitcoin-database#821

[thecodefactory]

I am able to sync testnet4 with a config like this:

[peer]
identifier = 675223068

[bitcoin]
genesis_block = 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
milestone = 00000000da84f2bafbbc53dee25a72ae507ff4914b867c565be350b0da8bf043:0
#milestone = 0000000002368b1e4ee27e2e85676ae6f9f9e69579b29093e9a82c170bf7cf8a:123613 
checkpoint = 00000000da84f2bafbbc53dee25a72ae507ff4914b867c565be350b0da8bf043:0
#checkpoint = 0000000002368b1e4ee27e2e85676ae6f9f9e69579b29093e9a82c170bf7cf8a:123613 
proof_of_work_limit = 486604799
minimum_work = 000000000000000000000000000000000000000000000b6a51f415a67c0da307
bip9_bit0_active_checkpoint = 0000000012982b6d5f621229286b880e909984df669c2afabb102ce311b13f28:1
bip9_bit1_active_checkpoint = 0000000012982b6d5f621229286b880e909984df669c2afabb102ce311b13f28:1
bip9_bit2_active_checkpoint = 0000000012982b6d5f621229286b880e909984df669c2afabb102ce311b13f28:1

[forks]
time_warp_patch = true
block_storm_patch = true
difficult = false

[thecodefactory]

@pmienk What's the proper way to have PRs like this with dependent branches complete?

[evoskuil]

retarget = true

This is a default (mainnet), no need to include.