Avoid triggers to start before other elements

[pcnfernando]

Purpose

Fixes #1542

Approach

Avoid inheriting from ExternalReferencedHolder and move the triggers as the last elements to start before the sources.

[codecov-io]

Codecov Report

Merging #1543 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #1543      +/-   ##
============================================
- Coverage      67.9%   67.87%   -0.03%     
  Complexity       45       45              
============================================
  Files           648      648              
  Lines         27315    27322       +7     
  Branches       4297     4298       +1     
============================================
- Hits          18549    18546       -3     
- Misses         6969     6981      +12     
+ Partials       1797     1795       -2

Impacted Files	Coverage Δ	Complexity Δ
...main/java/io/siddhi/core/SiddhiAppRuntimeImpl.java	63.1% <100%> (+0.25%)	0 <0> (ø)	⬇️
...n/java/io/siddhi/core/config/SiddhiAppContext.java	86.79% <100%> (+0.51%)	0 <0> (ø)	⬇️
...n/java/io/siddhi/core/stream/output/sink/Sink.java	64.92% <100%> (ø)	0 <0> (ø)	⬇️
...ore/util/parser/helper/DefinitionParserHelper.java	75.51% <100%> (ø)	0 <0> (ø)	⬇️
.../siddhi/core/util/event/handler/StreamHandler.java	62% <0%> (-12%)	0% <0%> (ø)
...n/java/io/siddhi/core/trigger/PeriodicTrigger.java	65.38% <0%> (-11.54%)	0% <0%> (ø)
...main/java/io/siddhi/core/trigger/StartTrigger.java	61.9% <0%> (-4.77%)	0% <0%> (ø)
.../main/java/io/siddhi/core/trigger/CronTrigger.java	69.49% <0%> (-1.7%)	0% <0%> (ø)
...o/siddhi/core/aggregation/IncrementalExecutor.java	81.25% <0%> (+0.78%)	0% <0%> (ø)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2f0e800...24a0103. Read the comment docs.