Make fStartedNewLine an std::atomic_bool

While this doesnt really fix the race of adding timestamps
mid-logical-line, it avoids the undefined behavior of using a
bool in multiple threads.

src/util.cpp

@@ -259,7 +259,7 @@ bool LogAcceptCategory(const char* category)
  * suppress printing of the timestamp when multiple calls are made that don't
  * end in a newline. Initialize it to true, and hold it, in the calling context.
  */
-static std::string LogTimestampStr(const std::string &str, bool *fStartedNewLine)
+static std::string LogTimestampStr(const std::string &str, std::atomic_bool *fStartedNewLine)
 {
     string strStamped;
 
@@ -286,7 +286,7 @@ static std::string LogTimestampStr(const std::string &str, bool *fStartedNewLine
 int LogPrintStr(const std::string &str)
 {
     int ret = 0; // Returns total number of characters written
-    static bool fStartedNewLine = true;
+    static std::atomic_bool fStartedNewLine(true);
 
     string strTimestamped = LogTimestampStr(str, &fStartedNewLine);