Fix bugs in avro options "mapping" and "mapping_only" for ConsumeKafka.py.

[jcferretti]

Thanks to Jianfeng for finding this was broken. Manual test showing this works now:

The share_price_record schema is shown below:

cfs@erke 23:16:20 ~/dh/oss3/deephaven-core
$ cat redpanda/examples/avro/share_price.json 
{ "type" : "record",
  "namespace" : "io.deephaven.examples",
  "name" : "share_price",
  "fields" : [
      { "name" : "Symbol", "type" : "string" },
      { "name" : "Side",   "type" : "string" },
      { "name" : "Qty",    "type" : "int"    },
      { "name" : "Price",  "type" : "double" }
  ]
}

[jmao-denver]

I am approve the PR based on the fact that all of the changes are meant to add robustness to the code. I don't think it is a good investment of my time to try it with any amount of testing myself to verify the changes as the author clearly stated that he had to manually test the changes and we have CI checks to prevent regression.