osd: fix pg epoch floor tracking

If you call erase() on a multiset it will delete all instances of a value;
we only want to delete one of them.  Fix this by passing an iterator.

Backport: firefly
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2228 from ceph/wip-pg-epoch

osd: fix pg epoch floor tracking

Reviewed-by: Samuel Just <sam.just@inktank.com>