[SPARK-17350][SQL] Disable default use of KryoSerializer in Thrift Server

In SPARK-4761 / #3621 (December 2014) we enabled Kryo serialization by default in the Spark Thrift Server. However, I don't think that the original rationale for doing this still holds now that most Spark SQL serialization is now performed via encoders and our UnsafeRow format.

In addition, the use of Kryo as the default serializer can introduce performance problems because the creation of new KryoSerializer instances is expensive and we haven't performed instance-reuse optimizations in several code paths (including DirectTaskResult deserialization).

Given all of this, I propose to revert back to using JavaSerializer as the default serializer in the Thrift Server.

/cc liancheng

Author: Josh Rosen <joshrosen@databricks.com>

Closes #14906 from JoshRosen/disable-kryo-in-thriftserver.