Describe the enhancement

As mentioned by @LLFourn in #976 (comment):

... I think electrum really intends for you to use the merkle proof API. Best solution is to do that and attach confirmation time of each block to LocalChain.

Additional context

• This should be attempted after Reimplement Wallet, ElectrumExt and Esplora{Async}Ext with redesigned structures. #976 is merged.
  * This may require changing LocalChain.