Use proper Timestamp type to track time (informalsystems#855)

* Use proper Timestamp type to track time

* Handle error when converting from u64 to Timestamp

* Use more precise result when comparing the expiry between two timestamps

* Move timestamp module to root crate

* Fix import errors

* Add basic tests for timestamp

* Address review feedback

* Update changelog

CHANGELOG.md

@@ -6,9 +6,10 @@
 
 - [ibc]
   - Reinstated `ics23` dependency ([#854])
+  - Use proper Timestamp type to track time ([#855])
 - [ibc-relayer]
   - Change the default for client creation to allow governance recovery in case of expiration or misbehaviour. ([#785])
-  
+
 ### BUG FIXES
 
 - [ibc-relayer]
@@ -24,6 +25,7 @@
 [#811]: https://github.com/informalsystems/ibc-rs/issues/811
 [#854]: https://github.com/informalsystems/ibc-rs/issues/854
 [#851]: https://github.com/informalsystems/ibc-rs/issues/851
+[#855]: https://github.com/informalsystems/ibc-rs/issues/855
 
 
 ## v0.2.0