core, light: get rid of the dual mutexes, hard to reason with

light/lightchain_test.go

@@ -122,10 +122,10 @@ func testHeaderChainImport(chain []*types.Header, lightchain *LightChain) error
 			return err
 		}
 		// Manually insert the header into the database, but don't reorganize (allows subsequent testing)
-		lightchain.mu.Lock()
+		lightchain.chainmu.Lock()
 		rawdb.WriteTd(lightchain.chainDb, header.Hash(), header.Number.Uint64(), new(big.Int).Add(header.Difficulty, lightchain.GetTdByHash(header.ParentHash)))
 		rawdb.WriteHeader(lightchain.chainDb, header)
-		lightchain.mu.Unlock()
+		lightchain.chainmu.Unlock()
 	}
 	return nil
 }