scan_without_keychain and scan's names should be inverted

[LLFourn]

There are some naming problems in electrum and esplora code.

• scan should mean without a keychain
• scan_with_keychain should be the name of the one that scans with a keychain

Same applies to update_tx_graph_without_keychain and update_tx_graph in esplora (which I would likewise rename to scan and scan_with_keychain).

The problem with the current naming is that it makes it seem scan (with a keychain) is the simple default and that scan_without_keychain is the exception. But scanning with a keychain (stop_gap) is very inefficient and should only be used when restoring from seedwords. Mutiny wallet caused a lowkey denial of service attack on mempool.space because they were always scanning with keychain.

• it needs to be documented on scan_with_keychain that it should be used sparingly and explain the circumstances where it should be used.
• I would remove all the other arguments from scan_with_keychain other than the keychain_spks in both esplora and electrum. This emphasizes the special case that it should be used for.
• Perhaps we should change scan to sync instead since that's the nomenclature we have elsewhere.

[vladimirfomene]

@LLFourn do you think this can be tagged "good first issue"?

[LLFourn]

sure but it should be done with high priority anyway.