buf_LRU_free_block(): Free compressed-only pages and their
descriptors with buf_buddy_free().

buf_LRU_get_free_only(): New function for returning a block from
buf_pool->free if one is available.  Split from buf_LRU_get_free_block().

buf_buddy_alloc_zip(): Rename from buf_buddy_alloc_low() and make static.
Remove parameter "split".  Always try to split.

buf_buddy_free_block(): Rename to buf_buddy_block_free().

buf_buddy_block_register(): New function for registering buf_block_t
objects reserved by the allocator.

buf_buddy_alloc_from(): New function for allocating an object from a
bigger object, and putting the unused parts on the free list.

buf_buddy_alloc_clean_zip(): New function for allocating memory by
replacing an unmodified compressed page.

buf_buddy_alloc_low(): New function for allocating memory, either from
the free lists of compressed pages, from the global free list, or from
unmodified pages in the buffer pool.