Running long integration tests could result in `bitcoind` dropping the
connection inbetween calls, and since python-bitcoinlib does not
reconnect and/or retry, all subsequent tests would fail as well. This
patch switches to throwaway connections, each serving just one
request. It's easier than to reach into the bitcoinlib to reconnect
and reauth, but comes with some overhead, but I think it's acceptable
for the few bitcoin calls we actually perform.
By looking for 'Done loading' in the log output we should actually be
called after `SetRPCWarmupFinished` in bitcoind. Only then is it safe
to make RPC calls. This resulted in the test suite being a bit flaky.
