AddressSynchronizer: change to use scriptPubKeys instead of addresses

[SomberNight]

This changes the AddressSynchronizer to use scriptPubKeys ("SPKs") instead of addresses.

• will allow subscribing to scripts that do not have a corresponding address, e.g. OP_RETURNS, p2pk, or bare-multisig
  • in particular, this allows supporting the pk(), raw(), and top-level multi() output script descriptors
• the wallet still has addresses, but adb has spks now
  • this results in some awkwardness, e.g. see AddressSynchronizer.get_history, .get_balance, .get_utxos methods
    • I think this is acceptable
  • especially during initial history sync of a wallet, there are lots of address->spk conversions
    • I was seeing noticeably worse performance (CPU load) when syncing wallets, so I made some fixes on master:
      • 0037053, and especially 61f2654
    • now the performance looks comparable to old master