[#790] storage engine: Add common template of log messages

There is a need to keep track of each local storage change. Log messages are
the most convenient way to do it.

Implement function which writes log message about the completed writing
operation in storage engine.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/local_object_storage/internal/log/log.go

@@ -0,0 +1,26 @@
+package storagelog
+
+import (
+	"github.com/nspcc-dev/neofs-node/pkg/util/logger"
+	"go.uber.org/zap"
+)
+
+// a distinctive part of all messages
+const headMsg = "local object storage operation"
+
+// Write writes message about storage engine's operation to logger.
+func Write(logger *logger.Logger, fields ...zap.Field) {
+	logger.Info(headMsg, fields...)
+}
+
+// AddressField returns logger's field for object address.
+//
+// Address should be type of *object.Address or string.
+func AddressField(addr interface{}) zap.Field {
+	return zap.Any("address", addr)
+}
+
+// AddressField returns logger's field for operation type.
+func OpField(op string) zap.Field {
+	return zap.String("op", op)
+}