[#132] Add internal event to process cleanup

This event will be produced by new epoch handler. All
cleanups should be synchronous because smart contract can't
store inner ring votes forever. Therefore voting should be done
in similar time interval for all inner ring nodes.

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

pkg/innerring/processors/netmap/internal_events.go

@@ -0,0 +1,9 @@
+package netmap
+
+// netmapCleanupTick is a event to remove offline nodes.
+type netmapCleanupTick struct {
+	epoch uint64
+}
+
+// MorphEvent implements Event interface.
+func (netmapCleanupTick) MorphEvent() {}