Various task updates relating to VM locking task:

• Break out lockmgr exclusive and sx/mutex tasks from one another, and leave the original (completed) movement of mutex pool initialization task as it's own thing.

• Create the task for moving to purely exclusive locks in VM rather than lockmgr shared locks. This task will involve implementation, then testing to see if it actually hurts. If this is possible, it will greatly simplify the transition to new locking primitives. Assign this to green since he's expressed willingness to work on it.

• Create the task for moving from lockmgr to either mutexes or exclusive sxlocks for VM. At that point, it should be possible to start pushing Giant down into VM somewhere, subject to lack of locking for a few primitives that will still require it, and for VFS/pmap interactions.