Moved evicter_t and eviction_bag_t to their own files.
Moved page.hpp to page_cache.hpp.
Moved page_t, page_acq_t, and page_ptr_t to page.hpp.
Made evicter.cc and eviction_bag.cc include page.hpp instead of page_cache.hpp.
Removed unnecessary predecl of page_acq_t.  Whatever.