Reduce memory footprint by enabling background page purging

Reduce memory footprint by enabling background page purging

Enable jemalloc background threads and reduce dirty page decay time from
10s to 1s so that unused memory is returned to the OS sooner.  As an
additional safety net, trigger a decay pass after every 16 MiB of frees
(rate-limited to once per second) to handle bursts that the background
thread might not catch in time.  On glibc, fall back to malloc_trim(0)
with the same volume-based trigger.