[identity] helper functions to interact with nonces table

Summary: we need to be able to get and remove nonces from the nonces table.

Test Plan: added a nonce to the nonces table. then inspected it with the get method and deleted it with the remove method.

Reviewers: jon, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8825