Reconstruct ChannelManager forwarded HTLCs maps from Channels

[valentinewallace]

We have an overarching goal of (mostly) getting rid of ChannelManager persistence and rebuilding the ChannelManager's state from existing ChannelMonitors, due to issues when the two structs are out-of-sync on restart. The main issue that can arise is channel force closure.

Here we start this process by rebuilding ChannelManager::decode_update_add_htlcs, forward_htlcs, and pending_intercepted_htlcs from the Channels, which will soon be included in the ChannelMonitors as part of #4218.

• test upgrading from a manager containing pending HTLC forwards that was serialized on <= LDK 0.2, i.e. where Channels will not contain committed update_add_htlcs
• currently, no tests fail when we force using the new rebuilt decode_update_add_htlcs map and ignoring the legacy maps. This may indicate missing test coverage, since in theory we need to re-forward these HTLCs sometimes so they go back in the forward_htlcs map for processing
• only use the old legacy maps if the manager and its channels were last serialized on <= 0.2. Currently this is not guaranteed