Use descriptor wallets for Bitcoin Core v0.21.0

[Sjors]

Coming soon(tm). I have some ugly WIP code to support it, but anyone should feel free to make a PR earlier.

I suggest checking the Bitcoin Core version and if it's 0.21 or later, leverage the following:

• createwallet takes a new descriptors argument, set it to true
• use importdescriptors instead of importmulti. It's the same syntax except:
  • you can drop keypool
  • you have to set "active": True for both receive and change
  • you have to set "internal": True for change
  • Nice benefit: the New Address button in Bitcoin Core GUI will work
• you can probably ditch all the keypool refill logic, as Bitcoin Core will just expand the descriptors as needed
• use balance = self.rpc.getbalances()["mine"] (instead of "watch-only")

[stepansnigirev]

What's the default address gap in core?

[Sjors]

The default gap is 1000, and can be configured with -keypool or (1 off) by setting a higher range during the descriptor import. If you set it to 1 million things get unacceptably slow, at least the last time I tried.

[ben-kaufman]

Closed with #737