node: RPC / FFI interface

[oleganza]

As the node implements the utxo index protocol (#426) and raw p2p protocol, we need a programmatic API to use it from other processes. You can also embed the whole node as a library, but for any other language, it will require an FFI, which would be effectively the same as this RPC API.

Core blockchain functions:

1. List blocks and transactions
2. List mempool
3. List peers
4. Connect/disconnect peers
5. Submit transaction

Tx builder:

1. Add predicate to watch (Index protocol #426)
2. Add utxo to watch and update utreexo proof
3. List utxos (can be done with decryption on client-side)
4. Reserve utxos (requires client to store them w/ decrypted flavor/quantity data)