Create possibility to generate large messages

[a-pejak]

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In order to test the performance of stream processing frameworks, we need messages of 1MB and 10MB with a rate of 500, 5000 and 50000 messages per second.

With the current implementation, we can use:

• randomContentString() function to generate string of predefined size, with size of 1000000 characters berserker was able to create ~60 messages per sec
• to use a predefined string of large size, with a string of 1MB it took 15 minutes to start producing messages, berserker was able to produce ~300 messages per second

Two possible approaches:

• use the content of a predefined file as value for the field
• only once generate the string of desired length and reuse it