Limit the number of pending un-funded inbound channel

Because we store some (not large, but not zero) state per-peer,
it's useful to limit the number of peers we have connected, at
least with some buffer.

Much more importantly, each channel has a relatively large cost,
especially around the `ChannelMonitor`s we have to build for each.

Thus, here, we limit the number of channels per-peer which aren't
(yet) on-chain, as well as limit the number of (inbound) peers
which don't have a (funded-on-chain) channel.

Fixes #1889

Author: TheBlueMatt