parachain-system (#296)

* rename parachain-{upgrade -> system}

* Merge message-broker into parachain-system

* Remove message-broker and clean up

* Update docs

* Test upward messages sending

And also update the relay-sproof-builder so that it allows to set the
relay dispatch queue size for the given parachain.

* Test horizontal message sending

* Remove old inherent definitions

xcm-handler/src/lib.rs

@@ -14,7 +14,7 @@
 // You should have received a copy of the GNU General Public License
 // along with Cumulus.  If not, see <http://www.gnu.org/licenses/>.
 
-//! A pallet which implements the message-broker APIs for handling incoming XCM:
+//! A pallet which implements the message handling APIs for handling incoming XCM:
 //! * `DownwardMessageHandler`
 //! * `HrmpMessageHandler`
 //!