Bound the size of `ChannelManager` internal HTLC holding maps

[ariard]

See #1835 (comment) A counterparty could still open thousands of channels during period of low-fees, and from then hold the HTLCs commitment on outbound links to blow up our memory.

[TheBlueMatt]

We need to take a systematic approach here. There's more trivial issues - a counterparty could simply open a million channels and never fund any of them and we'll probably OOM.

[TheBlueMatt]

I don't think this is super actionable - as a lightning node we're always bound by the rate at which a DoS attacker can put stuff on the chain - whether to open channels directly with us or to fill our network graph with channel entries. Its important that we have a constant factor space per on-chain transaction, but I think as of 114 we do, with the exception of the ChannelMonitor's historical state map. If there's some map that stands out as particularly bigger (say, bigger than 1KB/channel or 256B/HTLC) we should fix that, but I'm not aware of any such maps currently.