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.
This commit gives the current chainbackend the ability to connect and
disconnect the chain backend at will. We do this to let the chain
backend initiate the connection to the miner, not the other way around.
This is a preparation for using Neutrino as a backend, as it only allows
making outbound connections.
We must also move the setup of the chainbackend to after to miner, to
know the address to connect to.
