child_process: flush consuming streams

When a client calls read() with a nonzero argument
on a Socket, that Socket sets this._consuming to true.
It never sets this._consuming back to false.
ChildProcess.flushStdio() currently doesn't flush
any streams where _consuming is truthy. But, that means
that it never flushes any stream that has ever been read from.
This prevents a child process from ever closing if one of
its streams has been read from, causing issue #4049. This
commit allows consuming streams to be flushed, and the
child process to emit a close event.

Fixes: https://github.com/nodejs/node/issues/4049
PR-URL: https://github.com/nodejs/node/pull/4071


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>