Previously the chainbackend connected to the miner using a permanent
connection, which would retry the connection when it’s disconnected.
It would leave multiple connections between the chainbackend and the
miner, causing difficulties in debugging. Currently, there are two
occasions when disconnection happens. One happens when running the open
channel reorg test, the miner is connected/disconnected multiple times
on purpose. The other happens when the chainbackend receives a
MSG_WITNESS_TX type from the miner, which would be considered as an
invalid type and disconnects the miner. With the latter one being fixed
in btcd, the chainbackend will still be disconnected from the miner if
it reaches the ban score by requesting too many notfound messages in a
short time which is why the `--nobanning` flag is added.
