Modify launch script for iRkernel to delay SC initialization

[ckadner]

See issue #64: "Investigate async creation of spark session in launchers"

Modify launch_IRkernel.R script similar to launch_ipykernel.py (commit 6b0c2b7):

• initialize the Spark variables to generic placeholder object(s)
• "wrap" the initialize_spark_session() call in an asynchronous thread and start it just before calling IRkernel::main(connection_file)
• running a notebook cell that references any of the Spark variables will get "intercepted" by the placeholder object(s), which will wait for the initialization thread to complete (blocking) and then delegate the execution to the actual Spark objects
• after the initialization thread completed, the references to the placeholder object(s) will be replaced by references to the actual Spark objects