First step towards automating releases (T663). This eliminates manually copy-pasting the blockhash + chainwork.
Prerequisite: You must have bitcoind running with `--rest=1` set either as a CLI arg or in the config.
In the near future, we will have a dedicated node that can have these requests directed to (at that point,
the hostaddr default will change from localhost to that dedicated node).
Typical usage during a release is to replace mainnet params (`update-assumevalid.py` with no args set).
Occassionally, testnet params need to be updated as well, and may be done with `update-assumevalid.py -t`.
Both of these uses assume default node settings regarding rpcport, rpcusername, rpcpassword, etc.