Summary:
Fix a problem where `LoggerDB::get()` could crash when called after `main()`
has returned.

The `LoggerDBSingleton` object may have been destroyed already, so calling
`LoggerDBSingleton::getDB()` was not allowed.  This updates the code to just
store the singleton in a simple raw pointer, and only use the singleton helper
object for flushing log handlers.

Reviewed By: dmaone

Differential Revision: D13984692

fbshipit-source-id: a0c8550af367458ca39fefa9090e3165ad6a82bb