Use new filewriter package for logging to disk.

This allows for safe log truncation.

filewriter is an io.WriteCloser.  It takes a file path  and a buffer
size on initializing and proxies all writes to a file at the given path.

This allows for safe truncation as there is a lock in place ensuring no
data is written from the buffer until after truncation is finished, and
truncation can't run until the existing write to the file is finished.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>