[keyserver] add migration type to migrations
Summary:
This introduces the MigrationType which we can use to specify whether we want to wrap a migration in a transaction and block secondary nodes.
Depends on D13208
Test Plan: Tested later in stack
Reviewers: varun, ashoat
Reviewed By: ashoat
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D13209