Don't hold `channel_state` lock for entire duration of claim_funds

Don't hold `channel_state` lock for entire duration of claim_funds

When `claim_funds` has to claim multiple HTLCs as a part of a
single MPP payment, it currently does so holding the
`channel_state` lock for the entire duration of the claim loop.
Here we swap that for taking the lock once for each HTLC. This
allows us to be more flexible with locks going forward, and
ultimately isn't a huge change - if our counterparty intends to
force-close a channel, us choosing to ignore it by holding the
`channel_state` lock for the duration of the claim isn't going to
result in a commitment update, it will just result in the preimage
already being in the `ChannelMonitor`.