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:
https://github.com/LLNL/axom/blob/25744e566c650e8ce53c35f466672623eab1e917/src/axom/slic/streams/GenericOutputStream.cpp#L127-L138

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.

[rhornung67]

@kennyweiss based on @gberg617 comment, should this issue be closed?