Add je_iterate, je_malloc_disable, and je_malloc_enable

je_iterate calls a callback for every active jemalloc allocation in a
block of memory.  je_malloc_disable prevents allocations to avoid
mutating the jemalloc internal data structures while je_iterate is
running, and je_malloc_enable re-enables allocations.

Change-Id: I09c9554f36965806b47ecfce627560d1d9c33fda