[SPARK-2762] SparkILoop leaks memory in multi-repl configurations

This pull request is a small refactor so that a partial function (hence a closure) is not created. Instead, a regular function is used. The behavior of the code is not changed.

Author: Timothy Hunter <timhunter@databricks.com>

Closes #1674 from thunterdb/closure_issue and squashes the following commits:

e1e664d [Timothy Hunter] simplify closure