streaming should accept stderr from task before first key arrives

Stderr output from a streaming task is not collected until the MRErrorThread is started by PipeMapRed.startOutputThreads(), which is done on the first call to map() or reduce().

This makes it difficult to debug failures in starting up the task process. It can also lead to deadlock when a task receives no input keys but produces significant stderr output: the process will block on writing to stderr, while streaming will block waiting for the process to exit.

We should start the MRErrorThread when the process is forked, and then add the reporter later to enable stderr output serve as a keep-alive.