[tests] Fix abandonconflict.py intermittency #10344

abandonconflict.py intermittently fails (about 1 time out of 25 for me). The failure is caused by a race between shutting down node0 and a transaction being INV'ed from node0 and getting into node1's mempool.

The original intent was that node0 and node1 should be disconnected before sending the transaction in node0. However, the call to disconnectnode() doesn't actually disconnect the nodes because there are two P2P connections between node0 and node1 (connect_nodes_bi() madness).

This PR change the direct call to disconnectnode() into a call to the test_framework/util function disconnect_nodes() which fully disconnects the nodes. It also asserts that node1's mempool is empty after stop-starting node0. @TheBlueMatt

utACK 965a12442746e99247e5ce0f43fba8e3e149cae4 I imagine this should fix some random travis failures I've seen on a few of my PR, thanks.

Short version: ACK https://github.com/bitcoin/bitcoin/pull/10344/commits/965a12442746e99247e5ce0f43fba8e3e149cae4

Long one:

I have done thorough testing of this functional test script by 1000 runs on the master and this PR with

for i in `seq 1000`; do echo $i; ./abandonconflict.py > log.$i 2>&1; done

31 out of 1000 runs finished with:

2017-05-05 22:39:40.681000 TestFramework (INFO): Initializing test directory /var/folders/65/fn0h49r55k7779vg1b_h461r0000gn/T/testt_rlp1fk/46953
2017-05-05 22:39:55.575000 TestFramework (ERROR): JSONRPC error
Traceback (most recent call last):
  File "/private/tmp/bitcoin-master/test/functional/test_framework/test_framework.py", line 144, in main
    self.run_test()
  File "./abandonconflict.py", line 139, in run_test
    self.nodes[1].sendrawtransaction(signed["hex"])
  File "/private/tmp/bitcoin-master/test/functional/test_framework/coverage.py", line 46, in __call__
    return_val = self.auth_service_proxy_instance.__call__(*args, **kwargs)
  File "/private/tmp/bitcoin-master/test/functional/test_framework/authproxy.py", line 153, in __call__
    raise JSONRPCException(response['error'])
test_framework.authproxy.JSONRPCException: 258: txn-mempool-conflict (-26)
2017-05-05 22:39:55.576000 TestFramework (INFO): Stopping nodes
2017-05-05 22:40:00.121000 TestFramework (WARNING): Not cleaning up dir /var/folders/65/fn0h49r55k7779vg1b_h461r0000gn/T/testt_rlp1fk/46953
2017-05-05 22:40:00.121000 TestFramework (ERROR): Test failed. Test logging available at /var/folders/65/fn0h49r55k7779vg1b_h461r0000gn/T/testt_rlp1fk/46953/test_framework.log

This PR fixes this particular issue.

Other than this issue, there were 10 instances of TypeError: NoneType object is not iterable (https://github.com/bitcoin/bitcoin/issues/9317#issuecomment-284960894) and 5 instances of socket.timeout: timed out error.