Add notification channel to mutation manager

- Mutation Manager has a notification channel for ddl messages
from indexer. This fixes the synchronization problem for
concurrent ddl requests.

- Refactored the code so that StartMutationManager returns
an error if there is any issue in initialization
Send drop notification from indexer to mutation manager

- Use the mutation manager channel to send the drop/create
notification
- Use the new method for starting mutation manager
Remove unused keys.go

kv.go has the latest implementation for Key and Value interface