[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-encryption.py

@@ -30,8 +30,7 @@ class WalletEncryptionTest(BitcoinTestFramework):
         assert_equal(len(privkey), 52)
 
         # Encrypt the wallet
-        self.nodes[0].encryptwallet(passphrase)
-        self.bitcoind_processes[0].wait(timeout=BITCOIND_PROC_WAIT_TIMEOUT)
+        self.nodes[0].node_encrypt_wallet(passphrase)
         self.nodes[0] = self.start_node(0, self.options.tmpdir)
 
         # Test that the wallet is encrypted