thread_local: thread safety follow ups

[mattklein123]

A couple of follow ups to #12833:

1. Try to make the API a bit cleaner with templating: tls: simplify implementation and fix one class of crashing bug #12833 (comment)
2. Build clang-tidy plugin to check for any TLS functions capturing "this", and fix all current examples. (There are several that capture "this" in the initial set() call, and some that have been "fixed" to use shared_from_this and weak_ptr.)

[jmarantz]

Just to avoid the indirection I'll paste my referenced comment here with context:

to improve compile-time type safety, as well as less cumbersome usage, can we consider 2 changes:

1. Templatize the usage of slotes, so I put the fact that this is slotting a TlsCache into my declaration: ThreadLocal::SlotPtr<TlsCache> and then, for me, asType() is not needed.
2. Pass in object here as a non-const reference, so I can update if I like, but don't have to return it or declare the return-value.

Then one of the call-sites in source/common/stats/thread_local_store.cc changes from

    tls_->runOnAllThreads(
        [](ThreadLocal::ThreadLocalObjectSharedPtr object)
            -> ThreadLocal::ThreadLocalObjectSharedPtr {
          for (const auto& id_hist : object->asType<TlsCache>().tls_histogram_cache_) {
            const TlsHistogramSharedPtr& tls_hist = id_hist.second;
            tls_hist->beginMerge();
          }
          return object;
        },
        [this, merge_complete_cb]() -> void { mergeInternal(merge_complete_cb); });

to

  tls_->runOnAllThreads(
      [](TlsCache& tls_cache) {
        for (const auto& id_hist : tls_cache.tls_histogram_cache_) {
          const TlsHistogramSharedPtr& tls_hist = id_hist.second;
          tls_hist->beginMerge();
        }
      },
      [this, merge_complete_cb]() -> void { mergeInternal(merge_complete_cb); });

I think the Slot mechanism itself doesn't to change at all to accommodate this templating layer. It can do what it does now, and the templating API can be a thin header-only layer above.