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.

src/sync.cpp

@@ -37,11 +37,11 @@ struct CLockLocation {
         const char* pszFile,
         int nLine,
         bool fTryIn,
-        const std::string& thread_name)
+        std::string&& thread_name)
         : fTry(fTryIn),
           mutexName(pszName),
           sourceFile(pszFile),
-          m_thread_name(thread_name),
+          m_thread_name(std::move(thread_name)),
           sourceLine(nLine) {}
 
     std::string ToString() const
@@ -60,7 +60,7 @@ private:
     bool fTry;
     std::string mutexName;
     std::string sourceFile;
-    const std::string& m_thread_name;
+    const std::string m_thread_name;
     int sourceLine;
 };