Looking at the thread counts in our services, jemalloc's background thread
is useful, but mostly idle.  Add a config option to tune down the number of threads.

szind and slab bits are read on fast path, where compiler generated two memory
loads separately for them before this diff.  Manually operate on the bits to
avoid the extra memory load.

Add cast since read / write has unsigned return type on windows.

preserve_lru feature adds lots of complication, for little value.
Removing it means merged extents are re-added to the lru list, and may
take longer to madvise away than they otherwise would.

Canaries after removal seem flat for several services (no change).

1) make sure background thread 0 is always created; and 2) fix synchronization
between thread 0 and the control thread.

This reverts commit ae0f5d5c.

This completes the conversion; we now have only structured text output.

This was left over from a previous emitter conversion.  It didn't affect the
correctness of the output.

This is needed for things like mutex stats in table mode.

Another step in the conversion process.  The mutex is a little different,
because we we want to emit it as an array.

This makes the emitter the only source of callback information, which is a step
towards where we want to be.

This makes it so that the "general" portion of the stats code is completely
agnostic to emitter type.

While we're at it, print them in table form, too.

This is a step along the path towards using the emitter for all stats output.

The emitter can be used to produce structured json or tabular output.  For now
it has no uses; in subsequent commits, I'll begin transitioning stats printing
code over.

"always" marks all user mappings as MADV_HUGEPAGE; while "never" marks all
mappings as MADV_NOHUGEPAGE. The default setting "default" does not change any
settings.  Note that all the madvise calls are part of the default extent hooks
by design, so that customized extent hooks have complete control over the
mappings including hugepage settings.

test_alignment_and_size needs a lot of memory.  When percpu_arena is enabled,
multiple arenas may cause the test to OOM.

We have a buffer overrun that manifests in the case where arena indices higher
than the number of CPUs are accessed before arena indices lower than the number
of CPUs.  This fixes the bug and adds a test.

GCC on its own isn't quite able to turn the ticker subtract into a memory
operation followed by a js.

This seems to cause a configuration error with msys2.