[tests] Introduce TestNode

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.

test/functional/wallet-dump.py

@@ -94,8 +94,7 @@ class WalletDumpTest(BitcoinTestFramework):
         assert_equal(found_addr_rsv, 90*2) # 90 keys plus 100% internal keys
 
         #encrypt wallet, restart, unlock and dump
-        self.nodes[0].encryptwallet('test')
-        self.bitcoind_processes[0].wait()
+        self.nodes[0].node_encrypt_wallet('test')
         self.nodes[0] = self.start_node(0, self.options.tmpdir, self.extra_args[0])
         self.nodes[0].walletpassphrase('test', 10)
         # Should be a no-op: