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/node/runtime/src/constants.rs

@@ -34,7 +34,8 @@ pub mod time {
 	/// by `SLOT_DURATION`, but some slots will not be allocated to any
 	/// authority and hence no block will be produced. We expect to have this
 	/// block time on average following the defined slot duration and the value
-	/// of `c` configured for BABE.
+	/// of `c` configured for BABE (where `1 - c` represents the probability of
+	/// a slot being empty).
 	/// This value is only used indirectly to define the unit constants below
 	/// that are expressed in blocks. The rest of the code should use
 	/// `SLOT_DURATION` instead (like the timestamp module for calculating the