[keyserver] Extract query for checking the existence of personal thread

Summary:
This part of code has been exctraced in response to comments on D4941 (so it can be reused there)
Function searchForPersonalThread has been written in advance. It executes the query and returns it's result

Test Plan: Like D397

Reviewers: tomek

Reviewed By: tomek

Subscribers: ashoat, atul, abosh

Differential Revision: https://phab.comm.dev/D4977