epoch(9): always set inited in epoch_init

- set inited in the !usedomains case

Reported by: jhibbits
Approved by: sbruno

epoch(9): callback task fixes

- initialize the pcpu STAILQ in the NUMA case
- don't enqueue the callback task if there isn't sufficient work to be done

Reported by: pho@
Approved by: sbruno@

epoch(9): fix priority handling, make callback lists pcpu, and other fixes

- Lend priority to preempted threads in epoch_wait to handle the case
in which we've had priority lent to us. Previously

epoch(9): fix priority handling, make callback lists pcpu, and other fixes

- Lend priority to preempted threads in epoch_wait to handle the case
in which we've had priority lent to us. Previously we borrowed the
priority of the lowest priority preempted thread. (pointed out by mjg@)

- Don't attempt allocate memory per-domain on powerpc, we don't currently
handle empty sockets (as is the case on jhibbits Talos' board).

- Handle deferred callbacks as pcpu lists and poll the lists periodically.
Currently the interval is 1/hz.

- Drop the thread lock when adaptive spinning. Holding the lock starves
other threads and can even lead to lockups.

- Keep a generation count pcpu so that we don't keep spining if a thread
has left and re-entered an epoch section.

- Actually removed the callback from the callback list so that we don't
double free. Sigh ...

Approved by: sbruno@

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Add simple preempt safe epoch API

Read locking is over used in the kernel to guarantee liveness. This API makes
it easy to provide livenes guarantees without atomics.

Includes epoch_test kernel mod

Add simple preempt safe epoch API

Read locking is over used in the kernel to guarantee liveness. This API makes
it easy to provide livenes guarantees without atomics.

Includes epoch_test kernel module to stress test the API.

Documentation will follow initial use case.

Test case and improvements to preemption handling in response to discussion
with mjg@

Reviewed by: imp@, shurd@
Approved by: sbruno@

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