Don't create a `ChannelMonitor` for inbound channels until we have a confirmation

[TheBlueMatt]

If we get an inbound (non-zero-conf) channel, there's no reason to bother creating a ChannelMonitor until we see the funding transaction on-chain. Given monitors have relatively high cost, we should absolutely avoid this if possible.

[TheBlueMatt]

We should do this, but its a bigger lift. #3384 should get fixed in 0.2 and then this can wait until 0.3/0.4.