Add option to accept or reject inbound channels

Add option to accept or reject inbound channels

Add a new config flag `UserConfig::manually_accept_inbound_channels`,
which when set to true allows the node operator to accept or reject new
channel requests.

When set to true, `Event::OpenChannelRequest` will be triggered once a
request to open a new inbound channel is received. When accepting the
request, `ChannelManager::accept_inbound_channel` should be called.
Rejecting the request is done through
`ChannelManager::force_close_channel`.