The point is to decouple the key-value storage layer and the
event notification layer clearly. It gives the watchableKV the
flexibility to define whatever event structure it wants without
breaking the ondisk format at key-value storage layer.
Changes:
1. change the format of key and value stored in backend
Store KeyValue struct instead of Event struct in backend value for
better abstraction as xiang suggests. And record the corresponded
action in the backend key.
2. Remove word 'event' from functions
Snapshot takes an io.Writer and writes the entire backend data to
the given writer. Snapshot writes a consistent view and does not
block other storage operations.
Restore restores the in-memory states (index and book keeping) of
the storage from the backend data.
