util: avoid using thread_local variable that has a destructor

Store the thread name in a `thread_local` variable of type `char[]`
instead of `std::string`. This avoids calling the destructor when
the thread exits. This is a workaround for
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=278701

For type-safety, return `std::string` from
`util::ThreadGetInternalName()` instead of `char[]`.

As a side effect of this change, we no longer store a reference
to a `thread_local` variable in `CLockLocation`. This was
dangerous because if the thread quits while the reference still
exists (in the global variable `lock_data`, see inside `GetLockData()`)
then the reference will become dangling.

configure.ac

@@ -1049,11 +1049,6 @@ if test "$use_thread_local" = "yes" || test "$use_thread_local" = "auto"; then
           dnl https://gist.github.com/jamesob/fe9a872051a88b2025b1aa37bfa98605
           AC_MSG_RESULT([no])
           ;;
-        *freebsd*)
-          dnl FreeBSD's implementation of thread_local is also buggy (per
-          dnl https://groups.google.com/d/msg/bsdmailinglist/22ncTZAbDp4/Dii_pII5AwAJ)
-          AC_MSG_RESULT([no])
-          ;;
         *)
           AC_DEFINE([HAVE_THREAD_LOCAL], [1], [Define if thread_local is supported.])
           AC_MSG_RESULT([yes])