Fix image message bug

[nonumpa]

1. Fix image message bug, should only handle text.length >= 3
   Note : text.length < 3 won't be consider as new article sent in.

2. Limit the number of image messages linebot can process simultaneously.
   Too many image messages process simultaneously will easily cause messages timeout.

[coveralls]

Pull Request Test Coverage Report for Build 641

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.25%

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Totals
Change from base Build 631:	0.0%
Covered Lines:	219
Relevant Lines:	225

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💛 - Coveralls

[MrOrz]

Thanks a lot for the fix!

Let's make the management of imageProcessingCount flag more robust :D

[MrOrz]

If anything between imageProcessingCount++ and imageProcessingCount-- throws exception, imageProcessingCount-- will never be executed.

I suggest we do this to make sure imageProcessingCount-- is always called:

imageProcessingCount++;
try {
  // Anything between ++ and --
} finally {
  imageProcessingCount--;
}  

[nonumpa]

Updated, thanks for suggestion!!

[MrOrz]

LGTM!

[MrOrz]

It is working great on staging.

The concurrent number 3 is pretty good; when I pass 3 images to staging chatbot, I receive memory quota exceeded error, but since it's not "memory quota vastly exceed" that triggers restart, the 3 images do process in time. I believe we cannot raise the number anymore or we will use up all memory.