[services][identity] implement verify_user_token rpc

Summary: Query DB for token corresponding to user and device. If DB client returns a token, compare it to the one in the VerifyUserTokenRequest, checking if the strings match.

Test Plan: Called RPC using bloom and tested all paths

Reviewers: jimpo, karol-bisztyga, palys-swm, ashoat

Reviewed By: jimpo, karol-bisztyga, palys-swm, ashoat

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4199