logging: add LogFormatter and LogWriter interfaces

logging: add LogFormatter and LogWriter interfaces

Summary:
This simplifies the LogHandler interface to a single generic `handleMessage()`,
and adds a `StandardLogHandler` implementation that defers to separate
`LogFormatter` and `LogWriter` objects.

The `LogFormatter` class is responsible for serializing the `LogMessage` object
into a string, and `LogWriter` is responsible for then doing something with
the serialized string.

This will make it possible in the future to have separate `LogWriter`
implementations that all share the same log formatting code.  For example, this
will allow separate `LogWriter` implementations for performing file I/O
immediately versus performing I/O asynchronously in a separate thread.

Reviewed By: yfeldblum

Differential Revision: D5083103

fbshipit-source-id: e3f5ece25e260c825d49a5eb30e942973d6b68bf