Replace Akka Serialization with Spark Serializer and add unit tests.

Author: zsxwing <zsxwing@gmail.com>

Closes #7159 from zsxwing/remove-akka-serialization and squashes the following commits:

fc0fca3 [zsxwing] Merge branch 'master' into remove-akka-serialization
cf81a58 [zsxwing] Fix the code style
73251c6 [zsxwing] Add test scope
9ef4af9 [zsxwing] Add AkkaRpcEndpointRef.hashCode
433115c [zsxwing] Remove final
be3edb0 [zsxwing] Support deserializing RpcEndpointRef
ecec410 [zsxwing] Replace Akka Serialization with Spark Serializer