[#355] innerring: Refactor block timer constructors

This small refactoring adds `blocktimer.go` file with
all timer related function and constructors. This way
we can create all timers in one place (at the end of
innerring.Server constructor).

To do that we had to move timer reset into global
server state so it can be accessed by netmap
processor.

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

pkg/innerring/state.go

@@ -83,3 +83,10 @@ func (s *Server) WriteReport(r *audit.Report) error {
 
 	return s.auditClient.PutAuditResult(res)
 }
+
+// ResetEpochTimer resets block timer that produces events to update epoch
+// counter in netmap contract. Used to synchronize this even production
+// based on block with notification of last epoch.
+func (s *Server) ResetEpochTimer() error {
+	return s.epochTimer.Reset()
+}