Delay creating outbound ChannelMonitor until funding_signed rcpt

Delay creating outbound ChannelMonitor until funding_signed rcpt

Previously, we created the initial ChannelMonitor on outbound
channels when we generated the funding_created message. This was
somewhat unnecessary as, at that time, we hadn't yet received
clearance to broadcast our initial funding transaction, and thus
there should never be any use for a ChannelMonitor. It also
complicated ChannelMonitor a bit as, at this point, we didn't have
an initial local commitment transaction.

By moving the creation of the initial ChannelMonitor to when we
receive our counterparty's funding_signed, we can ensure that any
ChannelMonitor will always have both a latest remote commitment tx
and a latest local commitment tx for broadcast.

This also fixes a strange API where we would close a channel
unceremoniously on peer-disconnection if we hadn't yet received the
funding_signed, but we'd already have a ChannelMonitor for that
channel. While it isn't strictly a bug (some potential DoS issues
aside), it is strange that these two definitions of a channel being
open were not in sync.