[keyserver] Run migration to update thread pinned_counts

Summary:
Based on the changes made in D8683, we should also run a
safeguard migration to update the threads.pinned_count values to match
the number of messages in the database that are pinned for each thread.

Depends on D8683

Test Plan:

1. Pinned some messages
2. Set the threads.pinned_count = 0 to emulate a corrupt state that could occur because of this bug (invalid pinned_count value)
3. Run the query
4. Ensure that the pinned count values updated

Reviewers: atul, ginsu, tomek, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8732