[Post -0.21] Runing createwallet on a node should anyonecanspendismine it should rescan by default

With initialfreecoins and anyonecanspend in `elements.conf`, we should rescan the wallets for looking for anyonecanspend outputs.

Pre 0.21, this is not an issue because whenever the default wallet was created, it autoscanned the blockchain looking for anyonecanspend outputs. But since 0.21, there is no default wallet creation and using `createwallet` does not scan the blockchain for anyonecanspend outputs.

```
initialfreecoins=2100000000000000
anyonecanspendaremine=1
```

Quick fix: After creating a wallet with `createwallet`, in order to scan anyonecanspends run either `importprivkey `(tested) and ``rescanblockchain` or`(should work, but not tested).

If this is annoying/unergonomic to fix, we should at least update the documentation about this behavior. Notably, our tutorials from https://elementsproject.org/elements-code-tutorial/basic-operations won't work.