Startup on large nodes is slow

[rustyrussell]

@whitslack reported a 15 minute(!) startup time. Fortunately, he gave me a perf report, too:

-_start
               __libc_start_main
               0x7ff0cb565f59
               main
               |          
               |--91.30%--resend_closing_transactions
               |          |          
               |           --91.30%--drop_to_chain
               |                     |          
               |                      --91.30%--sign_and_send_last
               |                                |          
               |                                 --91.29%--wallet_extract_owned_outputs
               |                                           wallet_can_spend
               |                                           |          
               |                                           |--47.81%--scriptpubkey_p2tr_derkey

Sure enough, we loop through every key checking. This is silly, and we need to do better.