Add feature of output class to support an start_block_output() & stop_block_output()

[allevin]

Need an atomic output ability to the output class to allow a start_block_output() and stop_block_output() that collects output data. When stop_block_output() is called, all collected output data is then output. This is designed to prevent interleaving of output data.

Possible implementation with a counter that increments on each start and decrements on each stop, and outputs when counter reaches 0.

Requested by Scott H.