fix TODO
y -> n Kconfig
fix doc
Discard Cache Sharing

Really a big change.
According to the Mike Snitzer's advice.

- Now, device and cache is 1:1 relation. No cache sharing
- Admin tools are purged at all
- persistent metadata are in little endian now
- Quick start scirpts are modified and became simple
- Documentation updated but not finished
- Things to do remained still