srml-staking: Force minimum bond amount + proper cleanup. (#3048)

* enforce minimum bond amount.

* Bump.

* Improve doc.

* Update srml/staking/src/lib.rs

Co-Authored-By: Shawn Tabrizi <shawntabrizi@gmail.com>

* Update srml/staking/src/lib.rs

Co-Authored-By: Gavin Wood <gavin@parity.io>

* Doc update.