Add capability to choose networks for server and client.

Added capability to pass in testnet, regtest or mainnet parameters to the server. Also, added an initial modal to the client asking which network to use.

In the final version (for the client specifically), it might not be needed: default to BTC network. Mainly for easier testing and switching between networks.

Different wallets are also created for each variation.

I also commented out the checkpoints. If running on TestNet3 it didn't download any blocks.