When an opcode is about to be processed its dependencies are
evaluated using “_JobDependencyManager.CheckAndRegister”. Due
to its nature that function requires a lock on the manager's
internal structures. All of this happens while the job queue
lock is held in shared mode (required for the job processor).

When a job has been processed any pending dependencies are re-added
to the job workerpool. Before this patch that would require
the manager's lock and then, for adding the jobs, the job queue
lock. Since this is in reverse order it will lead to deadlocks.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>