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.
This commit moves functions start_node, start_nodes, stop_node and
stop_nodes functions into the BitcoinTestFramework class. It also moves
the bitcoind_processes dict and coverage variables into BitcoinTestFramework.
