Background threads may run for a long time, especially when the # of dirty pages
is high. Avoid blocking stats calls because of this (which may cause latency
spikes).
see https://github.com/jemalloc/jemalloc/issues/1502
cherry picked from commit 1a71533511027dbe3f9d989659efeec446915d6b
problems fixed:
* failing to read fragmentation information from jemalloc
* overflow in jemalloc fragmentation hint to the defragger
* test suite not triggering eviction after population
The original jemalloc source tree was modified to:
1. Remove the configure error that prevents nested builds.
2. Insert the Redis private Jemalloc API in order to allow the
Redis fragmentation function to work.
