WIP (persistent)

add simple interfaces for persistent, using leveldb
TODO: sync instance tomorrow
Merge pull request #86 from yifan-gu/persistent

WIP (persistent)
WIP

add Pack, Unpack for instance
wip

finish marshal/unmarshal single instance
finish marshal group of instances
feat persistent store

finish simple persistent storage module for replica
fix

fix udp encoder/decoder
TODO: change to TCP later
livetest(add timeout test)
change commands for demo
Merge pull request #87 from yifan-gu/persistent

Persistent
fix README