Removed unimplemented (and nonsensical) page_t::is_deleted.
Wrapped the destroy_ptr_ bool in page_loader_t.
Made page_t construction defer loading with block id, by only getting the block token, with a catch-up operation when a waiter is added.
Made the_page_for_read_or_deleted load page deferred, others load page for read.
Removed a bunch of now unused cache_account_t parameters.
Removed load_immediately_t and load_deferred_t parameters, since they are redundant with the cache_account_t parameter.
Merge branch 'sam_just_block_token' into next