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.
This work is prerequisite to attaching thread names to log lines and deadlock
debug utilities. This code allows setting of an "internal" threadname per
thread on platforms where thread_local is available.
This commit also moves RenameThread() out of a more general module and adds a
numeric suffix to disambiguate between threads with the same name. It
explicitly names a few main threads using the new util::ThreadRename().
