Allow plugin writers to create tasks that locks all of Pulp (within a domain)

[gerrod3]

On my work to implement #3358 I need a way to lock all of Pulp and prevent any other tasks from running while the migration occurs. In essence putting Pulp into a read only mode while the task runs.

We already semi-have this functionality with domains, as the domain href is added as a shared resource to each task in that domain. This prevents breaking a running task when updating/deleting a domain, as the domain will only be updated after all other tasks finish. We could enable this at all times even when domains are not enabled (as there is always a default domain) and allow plugin writers to exclusively lock on the current domain to create a 'global(domain)' lock on Pulp.