Make message macros (FAIL, WARN, INFO, etc) thread safe

This builds on the existing work to make assertion thread safe,
by adding an extra synchronization point in the holder of
`ReusableStringStream`'s stream instances, as those are used to
build the messages, and finishing the move of message scope holders
to be thread-local.

src/catch2/catch_message.cpp

@@ -101,8 +101,9 @@ namespace Catch {
     }
     Capturer::~Capturer() {
         assert( m_captured == m_messages.size() );
-        for ( size_t i = 0; i < m_captured; ++i )
+        for ( size_t i = 0; i < m_captured; ++i ) {
             m_resultCapture.popScopedMessage( m_messages[i] );
+        }
     }
 
     void Capturer::captureValue( size_t index, std::string const& value ) {