Add long-term-storage hooks to allocator API, implement for jemalloc

Add two new hooks to the allocator hooks API: alloc_long_term_arena()
and free_long_term_arena(). These allow the client to make allocations
from a dedicated long-term region, which can assist in minimizing
fragmentation due to mising of short-term and long-term
objects.

Implement this inteface for jemalloc, using it's arena APIs.

Change-Id: I62db22c2f197a980d81905d9f7c0da9e95e3af92
jemalloc: Show per-arena stats in allocator stats

Now multiple areans are actually used by memcached, print out each
arena's information as part of jemalloc's get_detailed_stats() call.

Change-Id: I5c2628952aeedd8ef2a27175736013da3c83564f