Convert epoch to read / write records per cpu

In discussing D17503 "Run epoch calls sooner and more reliably" with
sbahra@ we came to the conclusion that epoch is currently misusing the
ck_epoch API. It isn't safe to do a "write side" operation (ck_epoch_call
or ck_epoch_poll) in the middle of a "read side" section. Since, by definition,
it's possible to be preempted during the middle of an EPOCH_PREEMPT
epoch the GC task might call ck_epoch_poll or another thread might call
ck_epoch_call on the same section. The right solution is ultimately to change
the way that ck_epoch works for this use case. However, as a stopgap for
12 we agreed to simply have separate records for each use case.

Tested by: pho@

MFC after: 3 days