[Compaction] Improve EventListener::OnFlushCompleted() API and add listener_test

Summary:
Change the API of OnFlushCompleted() to include the name of the flushed column
family as well as the path to the newly created file.

Add several tests for EventListener.

Test Plan:
make listener_test
./listener_test

Reviewers: sdong, ljin, igor, dhruba

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D21855