IMPALA-2374: make AvroSchemaElement::ConvertSchema() run in linear time
IMPALA-2374: make AvroSchemaElement::ConvertSchema() run in linear time

In the case of nullable records, ConvertSchema() was recursively
calling itself twice, rather than once. This made ConvertSchema() run
in exponential time with respect to the number of nested nullable
records.

Change-Id: I39ba75076a348640bfb9a3d6e14948f6720cb0c8
Reviewed-on: http://gerrit.cloudera.org:8080/1123
Reviewed-by: Dan Hecht <20ebd99959f0f6e5dcb0fc319b4d8ef53a16b5fb@cloudera.com>
Tested-by: Internal Jenkins

michaelhkw/incubator-impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,cloudera/Impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,michaelhkw/incubator-impala,cloudera/Impala