i#4334: Fix leaks on thread exit

[derekbruening]

These leaks are all cleaned up at process exit, which is why the
existing exit-time leak checks don't notice anything. The global
block list lets us clean them up later, and hence a classic
reachability-based leak detector would not catch them either: so
perhaps they should be called "reachable accumulations" instead of
"leaks".

Fixes a memory leak on thread exit in all builds:

• The private reachable_heap units were not being freed

Fixes a number of memory leaks on thread exit in release build:

• Adds individual private fragment deletion on thread exit, to free
  stubs if -separate_private_stubs is on.
  Turns off -separate_private_stubs by default, to avoid this for
  shared caches where there are few benefits to separating private stubs.
  -thread_private turns it back on where the benefits probably outweight
  the thread exit costs.
• Moves the privload_tls_exit call to release build too to properly
  unmap the TLS.
• Moves the sigaltstack free to release build to properly free it.
• Moves freeing of "local unprotected" heap to the release-build path,
  since it is actually global! This is done for kstats, stats, and
  clone_tls.
• Moves client_data_t, client_todo_list_t, and thread-private
  client_flush_req_t to be PROTECTED to make it thread-local and thus
  not need freeing (the free is avoided for i#271).

Adds an -rstats_to_stderr dump point when DR terminates for OOM to
make it much easier to diagnose what the problem is.

Adds a new api.thread_churn test which attaches twice, once with a few
threads and once with many threads, and confirms memory usage has not
gone up. The 5 rstats on peak vmm block sizes are added to dr_stats_t
to facilitate this.

I tried to get api.thread_churn to work on Windows but hit a number of
issues with reattach. I solved two of them before bailing and making
the test UNIX-only for now and filed the rest as #4349.

• Unprotect .data on reattach
• Clear the logfile flag on detach

Issue: #4334, #271, #4349
Fixes #4334

[derekbruening]

The failures look like flakes: fib-conflict (#3406) and client.winxfer (#4058)

[johnfxgalea]

LGTM