Use milliseconds for timestamp resolution (#3210)

* node: tabify constants file

* node: define minimum period and slot duration in milliseconds

* core: srml: use milliseconds for timestamp resolution

* core: update slot_duration to millis in tests

* node: bump spec_version

* node: fix integration test

* node: fix executor test

* Update node/runtime/src/lib.rs

Co-Authored-By: Kian Peymani <Kianenigma@users.noreply.github.com>

* node: fix docs on timestamp resolution

* node: add docs on u64 for millis

substrate/node/primitives/src/lib.rs

@@ -50,7 +50,9 @@ pub type Index = u64;
 /// A hash of some data used by the chain.
 pub type Hash = primitives::H256;
 
-/// A timestamp: seconds since the unix epoch.
+/// A timestamp: milliseconds since the unix epoch.
+/// `u64` is enough to represent a duration of half a billion years, when the
+/// time scale is milliseconds.
 pub type Timestamp = u64;
 
 /// Digest item type.