[identity] get all usernames and wallet addresses in get_all_usernames

Summary: we should return wallet addresses in the function too so that we don't accidentally add claimed wallet addresses back to the reserved usernames table

Test Plan: call add_reserved_usernames RPC with a registered wallet address, address filtered out and not added to reserved usernames table

Reviewers: will, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12831