[#240] logs: Move log messages to constants

Drop duplicate entities.
Format entities.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>
Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/local_object_storage/writecache/mode.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"time"
 
+	"git.frostfs.info/TrueCloudLab/frostfs-node/internal/logs"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/shard/mode"
 	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/local_object_storage/util/logicerr"
 )
@@ -59,7 +60,7 @@ func (c *cache) setMode(m mode.Mode) error {
 	// flushCh is populated by `flush` with `modeMtx` taken, thus waiting until it is empty
 	// guarantees that there are no in-fly operations.
 	for len(c.flushCh) != 0 {
-		c.log.Info("waiting for channels to flush")
+		c.log.Info(logs.WritecacheWaitingForChannelsToFlush)
 		time.Sleep(time.Second)
 	}