Send our own HTLCs out via the forward_htlcs on-timer codepath

See-also https://github.com/rust-bitcoin/rust-lightning/pull/441#discussion_r406985733 but currently we forward HTLCs in batches on a timer, which is great for network-wide privacy, but we don't use the same batching for our own HTLCs. This is definitely not ideal and we should be using the same codepaths. We should also expand the same batch/timer code to include claims/fails of HTLCs.