Update to Bitcoin 18

[nopara73]

Bitcoin Core 18 is out: https://bitcoincore.org/en/releases/0.18.0/

The first blocker issue I encountered is that they removed the generate RPC (there may be more.) bitcoin/bitcoin#15492

generate is deprecated and will be fully removed in a subsequent major version. This RPC is only used for testing, but its implementation reached across multiple subsystems (wallet and mining), so it is being deprecated to simplify the wallet-node interface. Projects that are using generate for testing purposes should transition to using the generatetoaddress RPC, which does not require or use the wallet component. Calling generatetoaddress with an address returned by the getnewaddress RPC gives the same functionality as the old generate RPC. To continue using generate in this version, restart bitcoind with the -deprecatedrpc=generate configuration option.

We need to add to NBitcoin and make the Generate and GenerateAsync do getnewaddress before generatetoaddress so we can keep compatibility.

[lontivero]

We already did this in NBitcoin months ago: https://github.com/Metacosa/nbitcoin/blob/master/NBitcoin/RPC/RPCClient.cs#L1802

[nopara73]

Thanks @lontivero! This seemed to was the only blocker.