Rename methods of `EsploraExt` and `EsploraAsyncExt`

[evanlinjin]

          This method should not be called `update_tx_graph` since in the context of that library it is not updating anything. It's returning transaction relevant to a bunch of filters. I'd just call them `scan_txs` and `scan_txs_with_keychains`.

Originally posted by @LLFourn in #1040 (comment)