Improve usability when connecting to a remote bitcoind rpc server

[shesek]

• [ ] Implement persistent storage (#4)

Bandwidth optimizations:

• [ ] Convert into the Electrum SPV proof format from the bitcoind merkleblock format (https://github.com/shesek/bwt/issues/36)
• [ ] Batch RPC calls to bitcoind (https://github.com/shesek/bwt/issues/5)
• [ ] Configurable maximum number of headers per blockchain.block.headers request

Tor support:

• [ ] Allow connecting to bitcoind through a proxy (https://github.com/shesek/bwt/issues/21)
• [ ] Out-of-the-box tor socks proxy support (related: https://github.com/shesek/bwt/issues/8)