lianad: fix check that rescan has started

[jp1ac4]

The startrescan command uses the response from bitcoind's listdescriptors RPC method to confirm that the rescan has started.

This check includes a string comparison that can fail in some cases (Taproot descriptors with a spending path having only a single key) as the descriptor string returned by bitcoind may contain both h and ' , whereas Liana's descriptor string only uses ' (as per the Descriptor::to_string() method).

This PR changes the check to first parse each descriptor string as a Descriptor object to avoid any h / ' mismatch issues.

[pythcoiner]

tACK 6b914fd w/ this (signet) wallet:

tr([124b2e76/48'/1'/0'/2']tpubDFiSVLQkSqEZCopN3gby4hNDE7PG15mUrA5rjJWcMEzHFRzLY7bNCgeoVewjerasVejUizH7AWiZAZMKc7GWM7e4cj3TpUhsqLpgY3Rikca/<0;1>/*,and_v(v:pk([124b2e76/48'/1'/0'/2']tpubDFiSVLQkSqEZCopN3gby4hNDE7PG15mUrA5rjJWcMEzHFRzLY7bNCgeoVewjerasVejUizH7AWiZAZMKc7GWM7e4cj3TpUhsqLpgY3Rikca/<2;3>/*),older(65535)))#wvchffyz

rescan fail on master & success on 6b914fd

[edouardparis]

utACK 6b914fd