Add a small extent cache in front of the PAC ecaches. Allocs and dallocs
that fit are served from per-shard SEC bins without taking the ecache
mutex; overflow falls through to the backing ecaches, including
ecache_pinned for pinned extents.
The feature is gated behind experimental_pac_sec_nshards (default 0,
disabled). To support independent HPA and PAC SEC instances,
sec_alloc/sec_dalloc/sec_fill take an explicit shard argument, with HPA
and PAC using separate TSD shard slots.
