Summary:
Add a flush() call to the LogHandler interface.  This is needed to implement
`FB_LOG(FATAL)` so that we can flush all LogHandlers before aborting the
program.

Reviewed By: wez

Differential Revision: D5189499

fbshipit-source-id: 75fa4d7e75ea26de5b7383bf7e8d073fb37e9309