[zircon][thread] Don't kill threads on handle close

This is a long awaited step on the lifetime
reformation referenced in ZX-820 and in ZX-1106

The basic concept, better stated in the bugs
is that threads running state is not modified
by the handle count, only by directly telling
the scheduler what to do with the thread
which naturally bottlenecks in either
thread_exit() or thread_kill()

-Update the tests that poked at this behavior.

-Update the documentation, which was already wrong,
even considering the code before the CL.

Change-Id: Ic86942e739991359a20201b996d81cafb75083f7

[docs] Add objects.md and objects/foo.md pages

Change-Id: I95b6a42330f8b9d3711c5c894aeaf88a9256f2de

[docs] lightly improve documentation

- some cleanup
- some placeholders
- update xrefs

Change-Id: Ic548a9b9d09a611bb2d982d46f45c1e4a05ebfc9

[docs][objects] Re-organize kernel object documentation

Also add some stub entries for previously undocumented stuff

Change-Id: I0ac90ac5753a691a2b155d308de89c92e49ada5d