Add. EventConsumer class. (#1741)

Rewrite MWI and CallFlow subscribe handlers based on EventConsumer class.
Also on my test VirtualBox/Debian system Lua function `os.clock` produce
very strange result(delta ~0.015 for 1 second) so I switch to `os.time`.
Now to to stop this background Lua scripts it possible send CUSTOM event
with subclass `fusion::XXX::shutdown`. Where XXX is `mwi` or `flow`.

Usage of EventConsumer class
```Lua
-- create new object with timeout one minute
local events = EventConsumer.new(60000)

-- bind to some FS event
events:bind("SHUTDOW", function(self, name, event) ... end)

-- bind to another FS event with subclass
events:bind("CUSTOM::fusion::mwi::shutdown", function(self, name, event) ... end)

-- handle timeout event
events:on("TIMEOUT", function(self, name) ... end)

--run event loop
events:run()
```

resources/install/scripts/resources/functions/interval_timer.lua

@@ -1,3 +1,15 @@
+-- absolute timer
+local fs_time if freeswitch then
+	local api = require "resources.functions.api"
+	fs_time = {
+		now        = function()   return api:getTime()      end;
+		elapsed    = function(t)  return api:getTime() - t  end;
+		ms_to_time = function(ms) return ms                 end;
+		time_to_ms = function(t)  return t                  end;
+	}
+end
+
+-- absolute timer
 local os_time = {
 	now        = function()   return os.time()                 end;
 	elapsed    = function(t)  return os.difftime(os.time(), t) end;
@@ -5,6 +17,7 @@ local os_time = {
 	time_to_ms = function(t)  return t * 1000                  end;
 }
 
+-- monotonic timer (not work on my test Debian system)
 local os_clock = {
 	now        = function()   return os.clock()     end;
 	elapsed    = function(t)  return os.clock() - t end;
@@ -12,13 +25,19 @@ local os_clock = {
 	time_to_ms = function(t)  return t * 1000       end;
 }
 
+local timers = {
+	freeswitch = fs_time;
+	time       = os_time;
+	clock      = os_clock;
+}
+
 local IntervalTimer = {} do
 IntervalTimer.__index = IntervalTimer
 
 function IntervalTimer.new(interval, timer)
 	local o = setmetatable({}, IntervalTimer)
 	o._interval = interval
-	o._timer    = timer or os_clock
+	o._timer    = timer and assert(timers[timer], "unknown timer: " .. timer) or os_time
 
 	return o
 end