Rationale: allow updating existing descriptors with `importdescriptors` command.
Currently if you run same `importdescriptors` command twice with a descriptor containing private key you will get very confusing error — `Missing required fields`. What happens is that Wallet tries to write imported private key to the disk, but it exists already so we get `DB_KEYEXIST (-30995)` from BerkelyDB. Please note, that we set `DB_NOOVERWRITE` (I guess not to lose some keys accidentally). The exception is caught in `catch (...)` in rpcdump.cpp with a generic error.
With this PR if a descriptor is already present than we will update its activeness, internalness, label, range and next_index.
For the range only expansion is allowed (range start can only decrease, range end increase).
> wallet: allow to import same descriptor twice
> wallet: don't mute exceptions in importdescriptors
> wallet: maintain SPK consistency on internal flag change
> test: wallet importdescriptors update existing
> wallet: deactivate descriptor