[Minor][streaming][MQTT streaming example] some trivial fixes

[prabeesh]

modified to adhere to accepted coding standards as pointed by @tdas in PR #3844

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[srowen]

I'm not sure about most of these.

• Why is tempDir a field?
• _ is more idiomatic in Scala than null for a reference field's default value.
• Why remove the call to disconnect? almost seems like it should be in a finally block
• The regex change alters the functionality. I think "word count" examples across the world split on space.
• It's probably fine to resume one MqttMessage int he loop but are you sure?

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[prabeesh]

• the hard coded '\tmp' may create garbage. so created temp directory using internal methods and deleting after the use.
• revert to _
• added the finally block
• revert to " "
• In while loop, the MqttMessage object is getting created so many times. here just send same messages repeatedly

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[srowen]

@prabeesh my question about /tmp was why it is not a local variable. About MqttMessage, my question is simply whether you know you can reuse the message object. I imagine you can, but it is not obvious.

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[tdas]

Its more scala-like to not have the type in case of a val. So
val peristance = new MqttDefaultFi....

Also, peristance --> persistence !!!!

[tdas]

I am okay not having a jira about this. But please update the title to have [streaming] and MQTT streaming example in it. Some trivial fixes makes hard for us to understand from the commit list about what this patch was about.

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