rgw: add realm epoch to RGWRealm and RGWPeriod

rgw: add realm epoch to RGWRealm and RGWPeriod

RGWRealm::epoch is incremented on each new period, and
RGWPeriod::realm_epoch tracks the realm epoch when it was created

this makes it easy to determine whether a given period comes before or
after another period, which is needed to build a view of the period
history.  it also makes it easy to detect forks in the period history,
where two different periods share the same realm_epoch

Signed-off-by: Casey Bodley <cbodley@redhat.com>