TestNode is a class responsible for all state related to a bitcoind node
under test. It stores local state, is responsible for tracking the
bitcoind process and delegates unrecognised messages to the RPC
connection.
This commit changes start_nodes and stop_nodes to start and stop the
bitcoind nodes in parallel, making test setup and teardown much faster.
Raise RPC_WALLET_NOT_SPECIFIED instead of RPC_METHOD_NOT_FOUND when a required
wallet filename was not specified in an RPC call.
Also raise more specific RPC_WALLET_NOT_FOUND error instead of
RPC_INVALID_PARAMETER in case an invalid wallet was specified, for consistency.
