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 <[email protected]>

* node: fix docs on timestamp resolution

* node: add docs on u64 for millis