Start triggers in goroutine

[lixingwang]

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[] enhancement request
[] feature request
[] support request
[] general question

Current behavior (how does the issue manifest):
Today engine starts all trigger at main goroutine, main engine goroutine will be blocked if one of trigger fails to release the main engine goroutine.
Expected behavior:
All triggers should be independent so is that possible to start all trigger in different goroutine? otherwise, we need document the behavior to let user avoid block main goroutine.
Minimal steps to reproduce the problem (not required if feature enhancement):

What is the motivation / use case for changing the behavior?

Please tell us about your environment (Operating system, docker version, browser & verison if webui, etc):

Flogo version (CLI & contrib/lib. If unknown, leave empty or state unknown): 0.X.X

Additional information you deem important (e.g. issue happens only occasionally):

[lixingwang]

Check with #178 to add this into documentation.