## What changes were proposed in this pull request?
* Implement (optional) use of KryoPool in KryoSerializer, an alternative to the existing implementation of caching a Kryo instance inside KryoSerializerInstance
* Add config key & documentation of spark.kryo.pool in order to turn this on
* Add benchmark KryoSerializerBenchmark to compare new and old implementation
* Add results of benchmark
## How was this patch tested?
Added new tests inside KryoSerializerSuite to test the pool implementation as well as added the pool option to the existing regression testing for SPARK-7766
This is my original work and I license the work to the project under the project’s open source license.
Closes#22855 from patrickbrownsync/kryo-pool.
Authored-by: Patrick Brown <patrick.brown@blyncsy.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>
6cd23482d1