[server] add, check, and delete one time keys with session instead of user ID

Summary: We should add, check, and delete one-time keys by session instead of user ID for the initial launch. Changed some types and functions to reflect this.

Test Plan: Create new database, try to register and log in a user

Reviewers: atul, palys-swm, def-au1t

Reviewed By: palys-swm

Subscribers: benschac, karol-bisztyga, def-au1t, ashoat, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D2933