There is chance of dead lock that the Python process is waiting for ending mark from JVM, but which is eaten by corrupted stream.

This PR checks the ending mark from Python in non-block way, so it will not blocked by Python process.

There is a small chance that the ending mark is sent by Python process but not available right now, then Python process will not be used.

cc JoshRosen pwendell

Author: Davies Liu <davies@databricks.com>

Closes #4601 from davies/freeze and squashes the following commits:

e15a8c3 [Davies Liu] update logging
890329c [Davies Liu] Merge branch 'freeze' of github.com:davies/spark into freeze
2bd2228 [Davies Liu] add more logging
656d544 [Davies Liu] Update PythonRDD.scala
05e1085 [Davies Liu] check ending mark in non-block way