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@