logging: add a StandardLogHandlerFactory helper class

logging: add a StandardLogHandlerFactory helper class

Summary:
This moves some of the FileHandlerFactory code out into a new
StandardLogHandlerFactory helper class.  This will make it easier in the future
to add new LogHandlerFactory implementations that create StandardLogHandler
objects.

In particular, I plan to use this soon to split FileHandlerFactory into two
separate classes: one for writing to files on disk and a separate class for
writing to stdout or stderr.

Reviewed By: yfeldblum

Differential Revision: D6494227

fbshipit-source-id: 52e24250d020d21a5395d2a68fa5bd40bb32fbd4