- 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

Previously, we'd keep retrying a connection until the channel closed.
With this change, we only retry connecting *if there's a watcher on the
subchannel connection state*. This ultimately means that if the
lb_policy doesn't care if the subchannel connects, it'll stop trying.

Fix potential use-after-free: connected subchannel gets destroyed after its been picked by an lbpolicy

reporting (and improve control flow) in on_writable() in tcp_client_posix.c.