rpc: add `getblockchaininfo` and `getnetworkinfo`

Adds two new info query commands that take over information from
hodge-podge `getinfo`.

Also some new information is added:
- `getblockchaininfo`
  - `chain`: (string) current chain (main, testnet3, regtest)
  - `verificationprogress: (numeric) estimated verification progress
- `getnetworkinfo`
  - `numblocksofpeers`: (numeric) number of blocks of peers, as estimated from version messages; same as shown in the GUI
  - `localaddresses`: (array) local addresses, from mapLocalHost (fixes #1734)