Summary:
Add an ImmediateFileWriter class that implements the LogWriter interface by
immediately writing data it receives to a file descriptor.

This LogWriter implementation can be used by users that want to ensure log
messages are immediately flushed to a file as soon as they are written, at the
expense of possibly having logging I/O block their normal process threads.

Reviewed By: wez

Differential Revision: D5083105

fbshipit-source-id: 15cd071834e1784fe50e1d6d6ce403b9bc907047