doc: strongly recommend MALLOC_MMAP_THRESHOLD_=131072 for glibc HEAD master

The 131072 byte lower bound was the old default before the
sliding mmap window was introduced in modern glibc malloc.
While the sliding mmap window was intended to be faster by
reducing syscalls, zeroing and kernel overhead, it is also prone
to fragmentation from allocation patterns seen in evented Perl
servers.

Individual allocations over 128K are rare in our codebase since
there aren't many messages this large, making any performance
impact tiny.  Furthermore, the reduction in fragmentation and
memory use will be a speedup for memory-constrained systems
since they can avoid swap and have more leftover for the page
cache.

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