Use hashed name for key instead of just the segments.

## Summary In the `x/name` module, for the name records, use a hashed version of the full name as the key instead of a hashed version of the segments. ## Problem Definition In the `x/name` module, when storing a name record, we use a key that's the result of extracting all the name segments and hashing them. This has two problems: 1) It's a lot of extra work for nothing. 2) It can cause collisions in the keys for different names. ## Proposal Since this is changing the keys used to read/write some records, it'll require a migration. Update `getNamePrefixByType` (in `x/name/types/keys.go`) to just get a hash of the full name (rather than splitting it and using the segments. You'll need a copy of the original function for the migration, though. Then, create a migration that looks up all the old keys, deletes the entry, and re-writes it using the new key. This migration should be done as a module migration (i.e. bump the module's consensus version etc.). ____ #### For Admin Use - [ ] Not duplicate issue - [ ] Appropriate labels applied - [ ] Appropriate contributors tagged - [ ] Contributor assigned/self-assigned