Unify ChainSync actions under one enum (follow-up) (#2317)

Get rid of public `ChainSync::..._requests()` functions and return all
requests as actions.

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Co-authored-by: Sebastian Kunert <skunert49@gmail.com>

substrate/client/network/sync/src/service/mod.rs

@@ -16,8 +16,8 @@
 // You should have received a copy of the GNU General Public License
 // along with this program. If not, see <https://www.gnu.org/licenses/>.
 
-//! `ChainSync`-related service code
+//! `SyncingEngine`-related service code
 
-pub mod chain_sync;
 pub mod mock;
 pub mod network;
+pub mod syncing_service;