blocks that contains uncompressed and compressed frames.  This patch was
designed by Heikki and Inaam, implemented by Inaam, and refined and reviewed
by Marko and Sunny.

buf_buddy_n_frames, buf_buddy_min_n_frames, buf_buddy_max_n_frames: Remove.

buf_page_belongs_to_unzip_LRU(): New predicate:
bpage->zip.data && buf_page_get_state(bpage) == BUF_BLOCK_FILE_PAGE.

buf_pool_t, buf_block_t: Add the linked list unzip_LRU.  A block in the
regular LRU list is in unzip_LRU iff buf_page_belongs_to_unzip_LRU() holds.

buf_LRU_free_block(): Add a third return value to refine the case
"cannot free the block".

buf_LRU_search_and_free_block(): Update the documentation to reflect the
implementation.

buf_LRU_stat_t, buf_LRU_stat_cur, buf_LRU_stat_sum, buf_LRU_stat_arr[]:
Statistics for the unzip_LRU algorithm.

buf_LRU_stat_update(): New function: Update the statistics.  Called once
per second by srv_error_monitor_thread().

buf_LRU_validate(): Validate the unzip_LRU list as well.

buf_LRU_evict_from_unzip_LRU(): New predicate: Use the unzip_LRU before
falling back to the regular LRU?

buf_LRU_free_from_unzip_LRU_list(), buf_LRU_free_from_common_LRU_list():
Subfunctions of buf_LRU_search_and_free_block().

buf_LRU_search_and_free_block(): Reimplement.  Try to evict an uncompressed
page from the unzip_LRU list before falling back to evicting an entire block
from the common LRU list.

buf_unzip_LRU_remove_block_if_needed(): New function.

buf_unzip_LRU_add_block(): New function: Add a block to the unzip_LRU list.