Having the same variable name as 'index' type
can be confusing and shadowing other variables.
This gives different variable names to 'index' variables.
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
Before this commit, it will return wrong create index, mod index.
It lets findGeneration return error when rev is at the gap of two
generations. This leads to the change of compact() code.
