compressed pages from the buffer pool.

Makefile.am: Add buf0buddy.h, buf0buddy.ic.

buf/Makefile.am: Add buf0buddy.c.

Introduce the constants BUF_BUDDY_LOW and BUF_BUDDY_SIZES.

buf_pool_t: Add zip_mutex and the lists zip_clean and zip_free[].

buf_page_get_mutex(): Return &buf_pool->zip_mutex instead of NULL.

buf_buddy_get_offset(), buf_buddy_get(), buf_buddy_get_slot(),
buf_buddy_alloc_free(), buf_buddy_alloc_free_low(): New functions.