[keyserver] Add function to clear identity metadata

Summary:
Partially addresses ENG-6545.
Added a function to clear user ID and access token from metadata table. It's going to be called when service authentication fails.

Test Plan: Called this function manually and verified in TablePlus that these rows were removed. Calling verifyUserLoggedIn() creates them again.

Reviewers: varun, kamil, ashoat

Reviewed By: varun, ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13957