Refactor some types in XCMP queue (#813)

* Rename InboundXcmpStatus and its related types

* Rename OutboundXcmpStatus and its relevant types

* Add documentation to the newly created types

* Ensure storage prefixes are kept the same

* Add code comments

* Revert storage name changes

* typo fix

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>