MINOR: threads/checks: Set the task process_mask when a check is executed

MINOR: threads/checks: Set the task process_mask when a check is executed

Tasks used to process checks are created to be processed by any threads. But,
once a check is started, we must be sure to be sticky on the running thread
because I/O will be also sticky on it. This is a requirement for now: Tasks and
I/O handlers linked to the same session must be executed on the same thread.