Separate distribution and signing in Wallet interface

**Feature description**

Refactor `core.Wallet` interface for v2.

**Is your feature request related to a problem? Please describe.**
Address distribution , transaction creation and signing are altogether in `core.Wallet` interface. Besides the last two are quite similar to muiti-wallet API .

**Describe the solution you'd like**
Separate them three wallet aspects so that specialized concrete types may be built.

**Describe alternatives you've considered**
Keeping wallet contract in v1 API creates friction and duplication when trying to move forward.

**Possible implementation**

- [ ] Just refactor , no new functionality need be added .
- [ ] Update Skycoin plugin to wallet v2 API