Each tests/browser/server.py grew from about 13 MB to 0.6-1 GB RSS over
a page-workflow shard while its Python heap stayed near 12 MB. glibc
raises its mmap threshold (up to 32 MiB) after a large free, so the
seeder's image-sized buffers came from per-thread malloc arenas, which
fragmented and kept their pages. The server now fixes the threshold at
glibc's 128 KiB start value (mallopt M_MMAP_THRESHOLD) before it starts,
so large blocks go back to the OS on free. No memory limit is set; off
glibc it does nothing.
Closes#516
