Generalize flushing console buffer

Summary:
This generalizes concept introduced in D3543.
It helps under HPHP and it is useful also in PHP for outputs of size 1 (lower than our buffer size).

Test Plan:

id(new PhutilArgumentParser(array('', '--recon')))->parseStandardArguments();
echo "a";
PhutilConsole::getConsole()->writeOut('b');

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3562