[kernel][dispatcher] Remove unneeded tracking of whether threads were woken

This tracking was intended to prevent unnecessary context switches,
but it's no longer effective. AutoReschedDisable is more effective
for doing that.

* Instead of passing reschedule=false in Event::Signal() and calling
thread_reschedule() later, we can pass reschedule=true and then
there's no need to call thread_reschedule() later. We can remove
the return value from Event::Signal() and other methods.

* ChannelDispatcher::MessageWaiter methods: remove unused return
values.

* Dispatcher: remove the kWokeThreads return flag.

ZX-1472

Change-Id: I2b7d88f5c4c703e15df60d8fdecb712d5d220005

[dispatcher][state_tracker] Move the state tracker into the dispatcher

Note that all access to the old StateTracker functionality is still
mediated by the has_state_tracker function. This is used to e.g. allow
only certain objects to be user signalled.

Change-Id: If9d0413eb131c6e7d70ea19c1ed928f7bf566cc8

[kernel][statetracker] Make OnCancel and OnCancelByKey take a const Handle*

Also mark one override of OnCancel as such.

Change-Id: Ic1fb0bebf490768848834db03a03a296321ff366

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[zircon][mxtl->fbl] Rename mxtl to fbl

Change-Id: Ie21b6498e1bfb0a7fa0315e40b9e5c3ee78646be

[lib/magenta] Rename kernel/lib/magenta to kernel/object

TODO: Rename magenta.{cpp,h} and other instances of "magenta" in this
module.

For MG-1091 "Rename kernel/lib/magenta"

Change-Id: I2abc316f543798e00d3a4d1c2c83195da26e6836