logging: split FileHandlerFactory into two classes

logging: split FileHandlerFactory into two classes

Summary:
Split FileHandlerFactory into separate FileHandlerFactory and
StreamHandlerFactory classes, where FileHandlerFactory only handles logging to
files by path name, and StreamHandlerFactory only supports logging to stdout or
stderr.

The primary motivation for this is to allow logging to stdout or stderr in some
cases without allowing arbitrary files to be opened by FileHandlerFactory.
This can be achieved now by registering StreamHandlerFactory but not
FileHandlerFactory.  This makes it safer to allow controlling logging
configuration via command line flags even in setuid binaries.

Reviewed By: yfeldblum

Differential Revision: D6494226

fbshipit-source-id: a3ec371ca4266424d07dff20be18e6e13c057593