jemalloc 5.0.x causes TokuDB/PerconaFT to assert on load

mallctl_f("opt.lg_chunk", ...) is failing in memory init with jemalloc 5.1.0.  It seems this was removed in 5.0.0 https://github.com/jemalloc/jemalloc/releases/tag/5.0.0 and 5.0 no longer operates with the concept of lg_chunk :

"opt.lg_chunk" (size_t) r-
Virtual memory chunk size (log base 2). If a chunk size outside the supported size range is specified, the size is silently clipped to the minimum/maximum supported size. The default chunk size is 4 MiB (2^22).

There is no equivalent replacement functionality.

 For this specific issue, lg_chunk is used by PerconaFT to set the threshold where allocations will switch to using mmap instead of the standard allocator.  It is reasonable to recognize the lg_chunk query failure and pick a rational value and continue on rather than failing and asserting.  For the older jemalloc, this value was 4MB by default.  We should  use 4MB in the case that the lg_chunk query fails.