logging: add AsyncFileWriter

logging: add AsyncFileWriter

Summary:
Add an AsyncFileWriter class that implements the LogWriter interface using a
separate I/O thread to write the log messages to a file descriptor.

This LogWriter implementation ensures that normal process threads will never
block due to logging I/O.  By default it will buffer up to 1MB of data.  If
log messages are generated faster than they can be written to the file, log
messages will be discarded once the buffer limit is exceeded.  The LogWriter
will emit a message into the file recording how many messages were discarded
where the dropped messages should have been.

The downside of this class is that unwritten log messages still in the buffer
will be lost when the program crashes.

Reviewed By: wez

Differential Revision: D5083107

fbshipit-source-id: c67226f4d0726675d480b03eae83a29c5c3431b2