Summary:
1. Add override keyword to overridden virtual functions in EventListener
2. Fix a memory corruption that can happen during DB shutdown when in
read-only mode due to a background write error
3. Fix uninitialized buffers in error_handler_test.cc that cause
valgrind to complain
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4375

Differential Revision: D9875779

Pulled By: anand1976

fbshipit-source-id: 022ede1edc01a9f7e21ecf4c61ef7d46545d0640