[keyserver][lib] don't generate one time keys on identity login

Summary:

1. added a new helper function that only retrieves the identity keys and prekeys
2. updated login.js to use the new helper function, only getting one-time keys if login fails (to use for identity registration)

Test Plan: logged in from keyserver and confirmed that no new one time keys were added to the one time keys table in ddb

Reviewers: bartek, marcin, kamil, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11360