gc: Put epoch on its own cache line
gc: Put epoch on its own cache line

This brings linkbench with shared st_nlink reference counts down to 1
L2 miss per stat from ~1.45 (and down to ~0 if st_nlink is disabled).

I'm not sure why epoch was getting thrashed, but it might be worth
looking in to.

bowlofstew/sv6,aclements/sv6,aclements/sv6,bowlofstew/sv6,aclements/sv6,bowlofstew/sv6,bowlofstew/sv6,bowlofstew/sv6,aclements/sv6,aclements/sv6