Counter in Generic Stream

[gberg617]

We might want to add a counter to GenericOutputStream to indicate if any messages have been logged since the last flush() and only call flush() when there are messages. Flushing w/ no messages might be expensive since it might need to open and append (nothing) to a file.

See:

axom/src/axom/slic/streams/GenericOutputStream.cpp

Lines 127 to 138 in 25744e5

	void GenericOutputStream::outputLocal()
	{
	openBeforeFlush();
	m_stream->flush();
	}
	//------------------------------------------------------------------------------
	void GenericOutputStream::flush()
	{
	openBeforeFlush();
	m_stream->flush();
	}

Originally posted by @kennyweiss in #1649 (comment)

[gberg617]

Currently, GenericOutputStream will open a file when using an ofstream only at the first write call. Calling flush shouldn't have performance implications if the ofstream buffer is empty.