Ensure timestamp config makes sense. (#2132)

* Timestamp period should be expressed as expected period

* Fix test

* Ensure value never illegal

* Rename

* Remove println

* Add default

* Comment

* Fix, bump and rebuild wasm

* Fix test

* Add TODOs

* Configure only minimum_period.

* Fix

* Update srml/timestamp/src/lib.rs

Co-Authored-By: gavofyork <github@gavwood.com>

* Update srml/timestamp/src/lib.rs

Co-Authored-By: gavofyork <github@gavwood.com>

substrate/srml/staking/src/mock.rs

@@ -218,7 +218,7 @@ impl ExtBuilder {
 			invulnerables: vec![],
 		}.assimilate_storage(&mut t, &mut c);
 		let _ = timestamp::GenesisConfig::<Test>{
-			period: 5,
+			minimum_period: 5,
 		}.assimilate_storage(&mut t, &mut c);
 		t.into()
 	}