srml: babe: add expected block time and epoch duration constants (#3241)

* srml: babe: add expected block time constant

* srml: babe: expose epoch duration constant

* node: bump spec_version

* core: don't use moment type in test-runtime

* babe: add docs regarding c parameter

substrate/core/consensus/babe/primitives/src/lib.rs

@@ -104,7 +104,10 @@ pub struct BabeConfiguration {
 
 	/// A constant value that is used in the threshold calculation formula.
 	/// Expressed as a fraction where the first member of the tuple is the
-	/// numerator and the second is the denominator.
+	/// numerator and the second is the denominator. The fraction should
+	/// represent a value between 0 and 1.
+	/// In the threshold formula calculation, `1 - c` represents the probability
+	/// of a slot being empty.
 	pub c: (u64, u64),
 
 	/// The minimum number of blocks that must be received before running the

substrate/core/test-runtime/src/lib.rs

@@ -362,10 +362,12 @@ impl srml_timestamp::Trait for Runtime {
 
 parameter_types! {
 	pub const EpochDuration: u64 = 6;
+	pub const ExpectedBlockTime: u64 = 10_000;
 }
 
 impl srml_babe::Trait for Runtime {
 	type EpochDuration = EpochDuration;
+	type ExpectedBlockTime = ExpectedBlockTime;
 }
 
 /// Adds one to the given input and returns the final result.