The main idea is that NotifyContractUpdate adds the ContractUpdate to
a map called unmerged. It is populated in Start by shallow-copying the
activeHTLCs map values (htlcSet). The htlcSets underlying maps are not
copied, and so unmerged will just contain pointers to them. This should
be fine since unmerged will not modify them. At the call-sites of
activeHTLCs, it is updated to include the unmerged sets. This happens
with a mutex and should not cause any data race, even though it is
copying the underlying map pointers. No persistence should be
necessary since on restart, activeHTLCs and unmerged will just be
populated again.
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