KafkaCDC JSON conversion is taking most of the CPU time

In MXS-4775 it was found that there are inefficiencies in the KafkaCDC event-to-json conversion pipeline. Here's the visualization of the profiling from MXS-4775.

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Here's a quick list of improvements that can be done based on some quick profiling of the code I did locally.

• The gtid_pos_t::to_string() function uses a std::stringstream to create the GTID string. Using mxb::cat() and std::to_string() might prove to be better. sprintf() could also behave better as it is known that the maximum length of a single GTID value is fixed.
• The UTF8 checks done by the JSON library show up in profiling. This could be avoided by using json_string_nocheck() but the strings need to be validated to be UTF for this to be reliable. If the strings are known to be UTF8, the checks can be avoided.
• Most of the time is spent in KafkaEventHandler::commit() where the JSON object is converted into a string. If the string form of the JSON payload is constructed in-place, the overall cost of the JSON conversion would be lower.