Add global pages stat

[Xuesen]

Hello,

This is about https://github.com/memcached/memcached/pull/1086

Memcached's memory is managed by slab classes. Each slab class has many pages. When one page is not used by slab, it will be moved to the global page pool (i.e., slab class 0) for later use.

The stats slabs command shows page count of active slab classes, and the total_malloced memory size.
And if we add global pool pages count, it will be more helpful for users to understand memory information, i.e.,pages of all active slab classes + pages of global pool = total malloced pages

e.g.,

stats slabs STAT 20:chunk_size 7104 STAT 20:chunks_per_page 147 STAT 20:total_pages 40 ## active pages STAT 20:total_chunks 5880 STAT 20:used_chunks 5826 STAT 20:free_chunks 54 STAT 20:free_chunks_end 0 STAT 20:get_hits 0 STAT 20:cmd_set 5826 STAT 20:delete_hits 0 STAT 20:incr_hits 0 STAT 20:decr_hits 0 STAT 20:cas_hits 0 STAT 20:cas_badval 0 STAT 20:touch_hits 0 STAT 24:chunk_size 17352 STAT 24:chunks_per_page 60 STAT 24:total_pages 15 ## active pages STAT 24:total_chunks 900 STAT 24:used_chunks 869 STAT 24:free_chunks 31 STAT 24:free_chunks_end 0 STAT 24:get_hits 0 STAT 24:cmd_set 55874 STAT 24:delete_hits 0 STAT 24:incr_hits 0 STAT 24:decr_hits 0 STAT 24:cas_hits 0 STAT 24:cas_badval 0 STAT 24:touch_hits 0 STAT active_slabs 2 STAT global_pool_pages 45 ## unused pages STAT total_malloced 104857600 ## total memory END

STAT 20:total_pages 40 + STAT 24:total_pages 15 + STAT global_pool_pages 45 = total_malloced 100 pages