- trace system is now thread safe when run with TSAN
- fix a race in client_auth_filter.c
- allow timer manager to run in single threaded mode for fuzzers

This reverts commit 79759fea, reversing
changes made to dc36f4df.

If the slices to unref array was expanded after returning a pointer to a
slice, that pointer becomes invalidated. Instead, allocate and store
pointers to slices in that array to keep stability.

This reverts commit 5e01e2ac.

- make closures know where they should be executed (eg, on a workqueue,
  or a combiner, or on an exec_ctx)
- this allows removal of a large number of trampoline functions that
  were appearing whenever we used combiners, and should allow for a much
  easier interface to combiner locks